SS has a BF and BM, you are not one of them,  You  are not going to be a second mother !

The fog will lift fast.  A almost 14 yo doesn’t not want another mother.  It’s not going to be a TV show or movie,  you are not going to be one big Happy Family!  You should be more concerned about how you will become the second class person in the triangle.  

Make sure you keep your place as number one, once you give that up, you will never get it back.  Now.  Do you have a list of house rules.  When is bed time.  Cell phone and other screen times.  Xbox time.  What does this kid have to do.  Clean his room, dishes, yard work,    What happens if he breaks the rules, a taking to then ice cream? Reall punishment ?  Grounding with out electronic, Grounding to his room with every toy there ?  Do he has to keep his grades up ?

You should do some reading on these boards and take notes.  Get everything settled before he moves in.  So there no surprises..  Is DH going to do most of the parenting. Is BM CS going to stop now?  Is BM going to paid you CS. Who is going to paid for the plane for visitation to and back BM.  Is there dates set in stone for visitation.  Starting date,   Return date  flight from Tx to. GA could be a few $300 if not more..

It’s  not going to be fun.  Please read before you jump in.  Keep us informed, and join the club no one want to be a member of 

Why is he coming to live with you guys against his wishes? (Not that he should get to choose, just wondering why he's changing homes).

Start from Day 1 letting DH know that SS is his responsibility.  Be kind to him, but don't take a role of authority with him, and don't take over parenting for DH. Given that the kid doesn't want to move, it's not going to be an easy transition. I would just stay out of it and be around, but not a parent of any kind.

Why is he changing primary custody from BM's home to your home? Regardless of the reason, you may need some family counseling to help him deal with the situation. If he's that resistant to it, I'd be afraid he might try to run away. 

Also, I think a counselor can help you set some ground rules, which you definitely are going to need going in. In addition, I think that you personally might benefit from some counseling as you navigate this situation. I think it's so interesting how SMs are frequently willing to jump in and play "mom," not realizing what resistance they're in for. You're not his BM, and he's had 13 YEARS of living primarily with BM, so I doubt he's going to willingly just start treating another person as his mother, no matter how well-meaning. And as the person whose efforts are rejected, it can be very soul-crushing. 

I wish you all the best in this situation. 

Why is he moving?  Is this court ordered?  Is he being quite difficult for his BM?  Why is his life being uprooted to a state and two people he's not used to or settled with?  I don't understand the upheaval.

Are they doing this officially through the courts?  Because if your husband was paying CS you'll want to make sure that it is officially stopped and that BM is paying CS to your household now.  The last thing you need on top of a young teenager who doesn't want to be there are any financial issues or resentment over finances.

Good luck - I think supporting your husband, being kind to the child but not getting too involved is the way to go.  Communication with your husband is going to be key, and I'd be on the lookout for a good counselor.

Poor kid. Custody is switching because BM doesn't want to parent anymore?

My mother did something similar with my younger siblings. She and my SF wanted to move out of state and let my siblings decide who they wanted to live with. My brother always wanted to live with Dad, so no biggie there. My sister, though? She had just started high school and didn't want to leave her friends. It was wholly unfair to make her choose.

If I were you, I'd stay in thr background and encourage SS get therapy. He's old enough to understand what is happening, and that anger is going to flash hot as he gets older. Even though he'll be angry at BM, he'll direct it at DH. He needs a third party that he can talk/vent to who can watch for signs of depression and anxiety due to abandonment.

Also, set some house rules with DH that you'd like everyone to live by. Shoes off at the front door, dishes in the dishwasher, clothes in laundry basket, etc. Child rearing rules fall on Dad. Household rules fall on both of you to set.

Your DH is likely going to Disney it go, especially if SS responds well to bribery. Once SS gets a whiff of Dad's guilt, he'll flex that to his advantage. Be a silent observer, and only bring up things to DH if you think SS is in danger emotionally or physically. Convincing Dad to let him out of taking out the trash is minor; just leave it to your DH to do (he'll eventually get tired of having to do SS's chores). But convincing Dad that "using pot helps relieve his anxiety" is pretty major. THAT warrants a discussion.

You likely know what your DH's weaknesses are as a person, so try and support him through that. You likely already do this by how you split up household responsibilities, or how you interact with the outside world. Just observe for a while, see how full-time parenting impacts those weaknesses, and see if there are ways you can support him in those avenues versus going directly to SS. Example: if your DH has time management issues and is struggling to do grocery shopping and get SS to sports/activities, etc, offer to take on grocery shopping. It lets him focus on the parent side and you focus on the household/relationship side.

Stay in the background and treat SS like you always have. If SS is disrespectful, immediately bring it to DH and act swiftly if it continues. Disengagement will be your friend, and you can be both engaged with SS and your DH in a relational aspect while being disengaged from a parental one. 

Good luck.

Is the kid can start running the whole show !!!   You can not stay in the backround,  your place is Co head of the home.  You Control everyday life.  What has to be done, How the day plays out.,What for diner ect,ect.  There has to be some type of rules, The rules must be in force . He 13 yo and most likely has no rules,  like bed time. Who controls the TV.  

Just the everyday stuff that can drive you crazy