Constant fighting

My husband and i have been married 6 years , he has a son from a previous relationship when he was younger and we have 2 kids together. Problem is , the only time we fight is when his son is around . The child is 10 but honestly does not follow directions, doesn’t try to help himself, lazy , and acts worse than our 2 year old . These are also problems he has at his mothers house , we have talked to the mom she agrees he needs to be tested bc he has also had issues getting in trouble at school etc but will not take him . So it puts my husband and i in a hard spot bc we only get him every other weekend and bc the issues he is having isn’t getting handled AT home, there is only so much we can do 4 days out of the month , we are firm , consistent w him but it’s a constant nightmare and honestly i can’t handle much more . I love my husband more than anything , but it’s taking a toll on our marriage bc it’s constant fighting when the child comes over bc he doesn’t do what he should or chooses to misbehave, disobey. Any advice is appreciated TIA