it's not "unethical" but it could be somewhat enmeshed.  Mom is perfectly capable of dealing with the homework herself, or if she's not, your bf can take the kids to the library.   No need to be in the ex's home.

When you tell him you don't like this, what does he say???

I think inappropriate would be the word, but yes I would agree. If the ex cant even handle homework without his help she probably shouldn't have custody. Or if he insists he needs to be that involved at her house then he should stay single.

Oh I dont think so.  He can pick them up and help them somewhere else.  

Why isn't their mother helping them with their homework on HER custody time?

This is overstepping on his part to be in his EXW home on her custody time.

If the kids have a question on a particular assignment, there are any number of ways they can skype, text, email etc.. to ask dad if mom is not available or doesn't have a clue.

If the kid is having THAT much of an issue with regular assignments.. the Teacher needs to know so that they can get extra help at school.

If he going to his EXs house to help with homework.  More likely playing Happy Family.   He not ready for another relationship.  This is not the man for you. His EX are coming before you, 

I remember the big arguments that my husband and I had when he propsed doing this. Well...he went and did it once and it turned out awfully...it ended up just being BM having a go at him, so he never went back.

OP, at the end of the day you can only express your opinion and offer alternative solutions (like going to the library, or a coffee shop or paying for a tutor). If your boyfriend is determined to keep up with the homework help at his ex's house then you can't stop him. You could choose to leave him, you could choose to spend your evenings with one of your ex's or to go and do something that you enjoy in that time. Your reaction is up to you. 

My ex used to do that to spend time with the kids during the week without all the packing up and driving. We also allow BM to come over and see the kids but she doesn't very often. She is more likely to take them out to dinner 

I think this would be confusing for the kids. I guess it would make it a lot easier for him since he wouldn't have to keep them overnight on a school night. Maybe he should live next door to them so he wouldn't even need to drive over there. 

Go with him next time!!! I bet homework gets done a lot faster if he says no pack all his things while he is "doing" homework......and walk away from that mess and save yourself from step parent life for a while!!