He's doing the right thing I reckon - you dumped him once so he's thinking that you stand a chance of doing it again. It's a defence mechanism, to stop himself from being rejected again. He has a lot of bad feelings attached to you and it will take quite a lot of work to get rid of them so he can trust you not to hurt him again. He's being perfectly reasonable.
If you really wanna have a go though, my advice is to act in a serious manner to convince him that you're genuine. By this I mean start from the beginning by asking him out for a drink, do the whole dating thing to see if you and him really hit it off again. I think that might work better than simply sleeping with him.
You really want to be in the position that he's seducing you rather than the other way round and the only way I can see that happening is to start again and give him the opportunity - ie asking him out on an innocent date. Make sure you tell him that it doesn't mean you're in a relationship again - it's just you wanna spend some time with him etc. That will let his guard down. HTH, good luck! Just don't hurt him again!!!!