if they are still bathing him. Then some well-timed stories of other similar situations, wait for your bf to say that's inappropriate, and then make the comparison.

Also, NEVER help the child yourself. That should be a dad's domain in the bathroom to teach his son "dude" things. You are asking for trouble if you step in.

My skids are 4 and 7.  They are both capable of showering, although we usually turn the shower on for them to make sure the temperature is right, and we usually check the younger one before she gets out to make sure she got all the shampoo out of her long hair (same in the tub).  In fact, when they make a mess in the bathtub, the consequence is that they have to shower in the stall shower for a week or so.  They prefer baths because they like to play in the tub, so swapping tub for shower is effective in getting them to be more careful.

Why do you care whether the kid takes a bath or a shower?  Is he splashing water out or making a big mess in the tub?

a nice hot bath 9 months out of the year (not in the summer). So does my youngest daughter. We wash our hair in the shower and then soak/read for awhile. I even include a glass of wine lol. 

I don't think there is really a need to "teach" a kid to shower beyond helping them with water temperature but I also don't think taking a bath is being a "baby." it is all about preference. And many kids have trouble with things like washing their hair at 7 so assisting with that is also acceptable for the parents. I would insist that said child clean up after himself though. 

I'm with the others - this is your BF's job. Obviously he doesn't care (you are the one asking) so let him decide.

Don't risk the boy going back to BM and telling her you touched him or something.

My SS used to shower daily but his hair always was greasy and smelled like dirty odiferous dog.  He didn't have BO but his head just reeked.

So one day I told him to go put his swimsuit on and to jump in the shower so I could watch what he was doing.  So, he jumps in the shower, grabs the bottle of shampoo, turns it upside down over his head and squeezes it.  Then proceeds to scrub his hair.  The only problem being that the shampoo bottle was empty.  He then grabbed the bar of soap in the soap dish and scrubs his body.   

After sitting there LMAO while he grumbled through his shower routine "I'm not a baby you know. I know how to shower!"...  I told him to grab his shampoo bottle and squirt some shampoo in his hand...... of course nothing came out.   So I went to our master bathroom and grabbed a new bottle of shampoo out of our linen closet and took it back to him and told him to never again just turn the bottle upside down over his head and to always put the shampoo in his hand first. He had not been using shampoo for months though he would always get all butt offended when his mom or I commented on his greasy smelly hair. "I SHOWER AND WASH MY HAIR EVERY DAY!"  Every week we would make a shopping list and ask him if he needed anything for his bathroom. The answer was always "No, I don't need anything."  He had been using the magic never ending bottle of shampoo for months.  The cabinet in his bathroom had several bars of soap in it and since he had to physically grab a bar to scrub he knew when he needed a new bar.

You would not think that a late teenaged kid would need to have supervised shower training.

I still give him shit about that to this day.  He is impeccably clean and very cognizant of his hygiene.  He always has been. He just was having a teen boy brain fart of monumental proportions.

That brain fart is still good for a good laugh when we get together with SS.  10+ years after the fact. "Dad! That was a long time ago! Leave me alone. Ha! Ha! Ha!"   When he visits I make it a point to hand him a brand new bottle of shampoo and tell him to make sure he squirts it in his hand before he puts it on his head.

I think I was taking baths mostly until I was maybe 12 or so... I mean.. showers to rinse off at the pool.. but at home.. still taking baths.. because when you are little.. baths are nice and soothing and you can get all under the water.  When my knees would get cold out of the water.. or my shoulders.. when I got bigger.. showering was better and warmer for me.. lol.

your DH should be cleaning up after the kid.. and he should be bathing in his own tub.  

This is much less to do with the kid, or even BM, than it is with your BF. At 7 years old I think he's still innocent and mostly a victim of poor, inconsistent parenting, and there's still time to steer him down a better path. I think you should have a sit-down talk with BF and tell him how much unfair responsibility he's putting on you since your opinion isn't being considered. I would outline to him the rules you have in mind, such as: kid needs to learn how to properly brush his teeth, put his dirty clothes in the hamper, etc., and most importantly, being respected when you ask him to do something (BF should be firmly shutting that down the moment kid talks back). I'd also tell BF he needs to address the bathroom issue because boys learn best by watching their dad. Let him know he needs to back you up, and to commit to backing you up. 

If he shuts down your opinions at all, I would tell him then it's unreasonable for you to be expected to help raise his son in that case. A parental unit needs to be united with both perspectives considered. Maybe you want to be involved, which I think is great, so you deserve to have a parental opinion, whether you've had kids of your own or not. (BTW never believe the BS of your perspective being irrelevant because you haven't directly created a kid. You've had life experience and that is worth a lot.) It's your house too and kids need to be taught the rules of the house by both authorities.