You need to tell him before or while is going on.
I once had this crazy catholic girlfriend that for whatever reason ( I think she was traumatized by the church), didn't like to kiss when having sex. Craziest thing ever, but because of her, for my next gf, when we had sex, I would not kiss her while having sex, because I got used to the last gf; finally this girlfriend was like, "why don't you ever kiss me when having sex", and honestly, I didn't even think about it.i was on automatic, and thank god this girl snapped me out of it. But if she hadn't told me, I wouldn't have thought about changing my behaviors.
So my advice to you is to tell this guy, 'hey, I want to try what does it feel to have you come on my belly." Simple as that cause what if maybe this guy got messed up by another crazy girl out there? :) Once he tries it, then say, oh, I really loved how that felt, can you do it again? is really that easy.