how do i tell a boy i don't want to date him without shattering him?

If you want sincerely to be friends, then offer to plan an outing together & make it real.

If you don't feel a friendship either, start by empathizing. Write a note -- a short one -- that says you're extremely flattered & you know how brave he's being to ask you out. But you're just not feeling it (or say you're stuck on someone else, if you are.) Don't inject drama, be respectful & matter-of-fact.