Your husband need to FIRST consider anything that the child says with a grain of salt; it's QUITE common for children to manipulate their parents against each other, to either take the focus OFF of them or to put the focus ON them and it sounds like a little of both as you've stated that Mom shows favoritism towards daughter and he could be wanting more attention and that son has misbehaved and he could be trying to get out of trouble.
Sex is a NORMAL part of life...it is NOT "abuse" unless of course the adults are engaged in said activity in front of the kids.
Adults disagree, sometimes loudly...ALSO a normal part of life.
Again...your HUSBAND needs to discuss this Mom and they BOTH need to speak to son.
As for going to jail...it's always a possibility if your husband continues to violate the court's orders.