Don't give into his tantrums. Simply walk away. He is testing you. Take parenting classes, read books about parenting. Educate yourself. Be sure to have routines. Have rules and stick to them.
When my children were little they would have breakfast, get dressed, brush their teeth, we would make the beds. I would then let them watch 1/2 hour of TV while I got dressed. We would then go out for a few hours. I would take them to the playground or the park or the mall or to a play group at the local church or rec center. We would them come home, have lunch and then they would take a nap. This is when I would do chores around the house. When they woke we would have a snack and we would work on one page of an educational coloring book. They would then play as I would get dinner ready. After dinner it was usually back to the playground for an hour and then bath, snack a 1/2 hour of TV and then bed. Of course as they got older we were fitting in dance lessons and sports and school but pretty much stuck with the same routine, just altered. Kids needs structure. Don't let a two year old rule the roost and they will try. Good luck to you!

We are talking a 2yo. He gets no choice. He does what he is told and he eats what is put in front of him. To not set these boundaries now will just lead to a completely intollerable kid later.

You are the adult, quit choosing to be intimidated by a 2yo.

If he flips a tantrum... stick him in his crib and close the door. Let him freek out in the privacy of his own space. Tell him when he calms down he can come out. If you remove him from any audiance when he throws a tantrum he will soon learn that he gets nothing out of freeking out except banishment until he chooses a more appropriate behavior.

Sometimes a swat to the rump may be needed to reconnect his brain with his behavior.

Lather, rinse, repeat.

And wine. Lots and lots of wine. For some reason wine filters out a lot of whine.

How you step-parent: STEP back and let the parent do the job.

I think this depends on where the other parent (who is not your SO) is, but you only do as much as the other parent allows AND as much as the other parent does themselves (considering if you aren't completely embittered or afraid of rejection.) If the second bio parent who you are not in a relationship with is completely absent, I would consider stepping in if your SO is on board with you being a parent, but your experience may vary. There are cases where I one hundred percent believe the stepparent can be a full on parent (who eventually adopts the child), but usually they don't come here, which sets up the red flag that your SO is probably doing something wrong that would make any road to being an equal adult in your household difficult.

That said, a two year old is a baby. They throw tantrums. Occasionally I feel like throwing a tantrum when my two year old is throwing a tantrum, but as other posters said, just walk away.

I will add, if it's not too much trouble, saying before you walk away, just once, "Once you calm down, we can talk." Their comprehension skills are better than their speech skills, but "talking" for a few sentences after gives them closure and understanding. I usually just say, "Were you upset? Why were you upset? Because I care about you, I want you to learn that we don't get things by crying because it makes life hard for your when you grow up." Then give a hug and go on your merry way. This is BEST CASE scenario and how I have always handled BD's tantrums. She knows her tantrum isn't going to work, but also knows her concerns have been voiced by me, and getting an explanation gave her closure as to why she's not allowed to have/do whatever she wants (instead of just: I said no and that's the only reason you need.) She rarely ever throws tantrums now (I actually can't remember the last time she did) at a little over 2 and a half and accepts it when I say either, "I'm sorry you can't have that" or "A little later".

Also, sometimes you saying no is just the straw that broke the camel's back that led into the tantrum. I found that if BD gets cranky now, it's usually one of three things: She's hungry, tired, or something is uncomfortable for her. So I go through pulling out crackers and peanut butter and some water/juice and having her eat at least one, putting her down for a nap, or checking her diaper. Very, very rarely is it because we just said no.

If you feel like you can't handle it (especially if you were never around kids or don't have kids of your own) then just walk away and let him cry it out.

When SS4 was that age and tried throwing outrageous tantrums we (DH worked nights then) would make him go get in his bed and explained to him that he could come back out when he was ready to use his words to talk about what he felt/was thinking about. Consistency is key to success. Outburst and put him in his bed, if he wails louder then simply close his bedroom door. Once he realizes he isn't getting attention from it he will stop and come out to talk to you guys or he'll stop when he forgets what he's pouting about and decided to get up to play in his room. It's not so much of a time out because we've always let SS get up when he feels like he's ready. The rule was simply: that's not how you get attention from us and you will not cause chaos by doing it. Oddly enough at two that's what they want most of all--attention! And of course like any other BM, the tantrums will get louder and more outrageous until you prove you're serious about this boundary. Now he's 4 and when we are in the store and he sees a kid acting out he usually points at them and say "Geez Max that kid needs a serious nap" I'm happy to say we never had "terrible twos" with him at our house and I think it's because me and DH BOTH were consistent in expectations of behavior. Good luck.

He has no choice in what he want to do, let him throw thoes tantrum's. When I first started parenting my SS was 3 and he has anger issues. I made sure to tell him it's okay to be angry but not for the right reasons. I would take him to his room , and tell him, " when your done with your fit, come talk to me. Then we can work something out. Crying band being angry will get you nothing" I ducipline as I see fit. I did pop him on his had once because he threw fix and hit me in the store about no getting candy a t 7am. NOPE, I took his little self in the car and popped the back of his hand. It was cold out and I know it stung. Shut the door and let him cry his butt off then continued to pump gas. He got quite untill I got back in the car then started again, so I turned up the radio to drown him out. It worked!!!!! Then I realized that spankings we're not my style of parenting someone else's child. So alternative methods like taking away ALL his possessions in his room except the bed and sheets. ( He hit a pregnant teacher at daycare) Daddy took care of that right away. His anger and non listening of female athourty stems from him not listening and lack of parenting from his bio mom. AND the fact that she left him when he was 2. 

This August he starts kindergarten and I'm going to see how that goes. He hasn't gotten into anymore fights with kids at daycare but hopefully he has grown out of it some. I'm sure councling will be in his future so he can talk with someone. 

As long as there is no bio mom to disturb his life, we are doing great now. Bio is in jail! 

Don't give a 2yearold choices. Let him act up but then make sure he knows there Is consequences  to his actions and stick with it an even follow through with the discipline.