How do I say thanks, but no thanks?

A buddy of mine from my last command and I were brainstorming about a comic that I’m wanting to create. I just wanted to pick his brain, him being a big comic nerd like I am.

However, he started pushing to let me help him pencil. This guy lives in Japan now, and I on the East Coast of the US. I’ve seen his drawing style, and we both agree he’s less consistent and versed in figure drawing than I am, and I’d rather keep him on for plot ideas than as a penciller.

Another thing to consider is his internet in Japan is bad, and it’ll be inconvenient for him to scan his art, then me fix it up (as he suggested).

How do I politely tell him no, when things like “I’d rather you as a writer” and “I don’t think that will work” don’t get through to him?