he says he wants to leave
There is absolutely nothing stopping him, so why doesn't he?
often texts and says bags packed.
Really? And you believe him? What does his wife say when she sees his packed bags?
but then nothing happens.
Of course it doesn't. Because he has no plans to leave his wife.
hasn't slept with his wife in 3 years
You have no way of knowing this other than his word. And by definition he is a liar (unless he has told his wife about you.) So I wouldn't put much stock in anything he says about his current wife. Personally, I think it is simply designed to keep you jumping into bed with him.
its loveless and he hates being there
Bullcrap. If he hated being there, then he would leave. Simple as that.
He has a son, says he stays for him.
Again, that is bull. While his son may not be happy seeing his father leave, he would get over it as long as his dad stays in the picture and sees him regularly. This is just an excuse to keep you jumping into bed with him.
so how long do I wait.
You don't. You stop screwing with another woman's husband. Tell him it is over until he leaves his wife. And mean it. If he knows it is over unless he leaves, but he chooses to stay with her, then you know you were being used. My bet is that he will let you go on your merry way and find some other willing woman to bed. After all, he is a cheater. It is what they do.