I think Coiled Serpent has proposed some great ideas here. Your boyfriend has found what works for him, to totally change the method in which he stimulates himself is a bit drastic, just to be 'normal' like everyone else. It's like, if you required your
clitoris for
orgasm and he were to say "no I don't want to touch that, getting off with the clitoris feels too good and makes you unable to get off through vaginal intercourse like normal people, so stop touching it and learn to like this".
It's like the 'good for the goose/gander' type thing.
Besides using outercourse, I think you can potentially have intercourse while still stimulating him this way. I'm pretty familiar with prone
masturbation since I myself practise that (hands free) as an alternative to the more normal supine form.
This may not apply to all guys, but for me personally, I point it down. It doesn't naturally go that way either, so it is rather uncomfortable if you try to just put it there, generally it's only possible if you ease into it while erecting from a flaccid state.
I don't see how people could do it prone any other way, honestly. The underside is much too sensitive and there's like a crushing force on it or something. If guys are doing it that way I could see how they could get problems, though not so much if done on a mattress or pillow. But if it's on a wooden floor, it has to point down or it'd hurt too much. This way, the sensitive underside doesn't touch the floor, it touches the sides and it's better.
So anyway, I saw this hilarious porn movie once where it looked like the missionary position, only rotated 180. Basically, instead of being lined up, they were reversed, as in feet to head and head to feet, rather than head to head and feet to feet.
It looked really odd, like some kind of yoga kama sutra type thing. But basically, this has the same effect I think, because it points downward, so it would give a similar (or actually, maybe even better type of stimulation.
It definately looks strange from afar though and I imagine is awkward to do (harder to balance than alone) and potentially dangerous, and probably less romantic (since it's like 69ing basically, only you're moved down so there's less/more ass visible).
Actually, based on that, since you're probably going for romance it may not be the ideal solution, but since you can 'spin' basically (probably something you gain experience with over time) you can switch between traditional and inverted missionary to please him, and to make the romantic connection you desire. It is exotic and you and he will feel like contortionists.