I'd say a good first step is to talk to him and get a feel for what he thinks about it. A lot of guys think that anything going near their butt is "gay" and although that's totally not true he might still think that. You may have to be very very patient and don't be too forceful, because that will probably just push him away. Let him know how good it can feel to have a prostate massage and how happy it would make you to try it on him. This will most likely be the biggest hurdle to clear.

Probably the most important thing I can advise is to start small and go slow! If he'll at least allow you to touch him down there, you can just massage the area, either directly or through underwear. When he's comfortable enough to go for penetration, start with just one finger and use PLENTY of good lube (there are a lot of good anal lubes on this site). And always remember not to rush. Hopefully then you can move up to more fingers and eventually some toys. I wouldn't do all this the first time, of course. Let him slowly warm up to it. Another thing I'd recommend is pick a time when he's more relaxed. If he just came home from a stressful day at work he'll be more tense, and if he gets hurt that will only put him off. If you want a little extra help, a good massage, hot bath, a little alcohol, or anything else that he finds relaxing may be a good idea.

I hope that helps, even just a bit. You can also do a bit of research on the net, I've read a ton of different articles about having enjoyable anal sex with your man. Good luck!