I'm going to pose a different point of view.

Your son is being encouraged by his father to show favoritism towards him. This, I can understand, must be terribly difficult for you.

However, if your son thinks it's such a paradise at his father's house - why don't you let him go there? I'd be suggesting 50/50 custody arrangements as a compromise. I'll bet that once he has to REALLY live with them (in her house, competing for attention with her kids), he'll get quite a different perspective.

Instead of fighting it, encourage it. Agree with your son that he may be better off spending more time with his father. Let your son work out the dynamics of each household by spending half his time in yours and half in theirs.

Things won't be such 'fun' when he's being disciplined by them, and I'll bet the old 'hell hole' won't look so bad. Things won't look so rosy when he takes off those rose colored glasses.