I have been trying for some time now to get my ex to sign over his parental rights. He never sees my son, we have been in and out of court for child support until recently he quit his job to get a job under the table. He owes a couple thousand dollars in child support arrears and I told his fiancé that if he would give up his parental rights I would wipe the slate clean and he wouldn't owe me any money. But I got no response from him. Is there anything further I can do in order for him to give up his parental rights?

Nope... at least not as long as he's not harming or endangering the child.

If you can prove to the court, though, that he's working "under the table" so he can claim to the court that he has no job; and then if he actually does so, he will have perjured himself... which can also lead to a contempt of court charge... both of which can end-up in jail time. Plus, at that point, the IRS would probably be interested... and one never wants to screw with the IRS!

If you're going to pass messages along to him through his current squeeze, you might let him know, through her, that if he doesn't straighten up, a fate such as that is what is going to befall him.

Also, be careful what you promise in the way of wiping-out arrears child support. In most states, that's not your decision to make. It's the child's money, not yours. If the court ordered it, then he has to pay it, no matter what. Occasionally, with large arrears, and when the child has finally reached the age of 18, the prosecutor, in consultation with the custodial parent, will strike an agreement for a severely reduced lump-sum payment (or sometimes time payments over a very short period of tijme) with a non-custodial parent who owes arrears child support. But, in any case, in most states that's generally not your shot to call.

No there isn't anything you can do. If he dosen't agree to sign over his rights then there is no deal. Besides even if he did agree to it you would have to convince a judge to allow it which is pretty hard to do. You're better off going back to court to try to get the support owed. Also I'd quit trying to communicate through his fiancée that would rub a judge the wrong way like you are tying to mess up his relationship they (judges) tend to be more sypathetic if you go about things through court or with the child's father with out making demands (emotions can get the best of you).

Unless you have a new partner willing to adopt, it is not going to happen the way you want. First if you or the child would ever had to get any public assistance, the state will and can go after him for repayment.
Next unless he would agree you just can't ( unless he is a threat or has been in prison or something)
Next so he is working under the table, you take him back to court, prove his life style, where he lives, what he drives and it will prove he is lying to the court. Plus even not working all states have a min amount that is payable, normally set off min wage. So start trying to get him thrown in jail or get his drivers license suspended what ever your state allow