Asking for what you would like sexually in a sexual relationship doesn't make you overwhelming or a nag (unless you keep pestering him about it after you've already addressed it together, say twice). Asking for what you want is just communicating your needs.

If you call him selfish and say 'hey, I'm always blowing you but you never go down on me,' that's hella improper. But if you ask, 'hey, I'd really like it if you went down on me, are you up for that?' there's nothing wrong with that.