I think the fact that you're willing to put up with the pain because you know topping makes your man happy is seriously sweet - but not at all practical or that much fun for you.
I think it's probably (cliche as it is) just to talk to him about it.
If you're willing to bottom, then there are loads of ways he can get you to relax more to let him in without hurting you. That's assuming you're a natural bottom/versatile.
I think a lot of men would love to top or bottom if they take the opposite role, but some simply can't bottom. One of my boyfriends is a total top, and the other a versatile bottom. The top sees me and the vers. have flip flop fun, and would love to try too, but in the past bottoming, despite the want and intention, wasn't happening, as it wasn't something he could do, and as such, it wasn't fun to know he was getting hurt.
There is something called the comecloser ring (cumcloser?) that I read about a while back.
Basically, it's like a 1-2 inch long cockring/tunnel-like toy that you put on your dick at the base, and when you fuck, the ring stops you from fully penetrating the ass (or vagina :]) whilse still making it feel that you're completely inside, as you have a physical resistance pushing back on the base of your cock, making it feel like you're fully inside.
If that is still around (I'm sure it is, but can't find any links sorry) then that might be something worth looking into. If it's too much to take, but you really want to bottom, and your man really wants to top, it could be something worth pursuing!
Wish I could be more help.
But hope all works out!
Keep us updated about how you get on! ^_^
