If your DSS is over the age of 12 than his mother can't force him to go back to live with her, even if she has custody. Once he is over the age of 12, he has a right to decide where he lives. So your DH very much can make a claim to have child support reassessed without the fear that his ex will force his son back. It's simple. She can't.

Just thought I'd clear that little misunderstanding up for you so that you can at least alleviate that particularly stress.