I guess we were just lucky. My daughter had no problem wearing a helmet at that age. Maybe because she loved Dora so much (a "Dora the Explorer" helmet)? Or maybe because she wanted to ride her "bike" (a big wheel at that point) so much.
But two-year-olds are called "terrible twos" for a reason (even if they're only 20 months). Good luck, I think if you keep at it he'll get used to it. And I completely agree with getting them started early.