+1 on the 'he has to really want it.'
Also, if he doesn't ever play with his own ass, that's a really important first step. Have him finger himself sometime and learn to relax a little more; he can repeat those muscle commands when he tries to bottom for you.
Something that works for me but not for everyone is to get fingered first. I find that getting a good coating of lube on my insides makes everything a lot easier. My first several experiences involved two or three fingers beforehand, just to make sure I'd relaxed enough.
Make sure he remembers to breathe! I know it sounds silly, but I frequently hear stories about how people are so concentrated on trying to get it in that they forget, and that causes them to tighten up.
Go really slow, and don't worry. If either of you worries, it's not gonna work out. If you get it in a little, hold it where it is and do something that turns you both on. Play with his balls or nipples, kiss his body, etc.
Lastly, something that works really well for me is to get to a point where I can't move any further down on my bf's cock, I relax and move about 1/4" up on it before sliding down slowly. I can usually take a little more of it this way, repeating until I get past the thickest part, and then I can slide the last few inches in all at once.