So don't ask him if he wants to sit on the toilet. MAKE him sit on the toilet. Of course he's going to say "no" if you "ask" him. He's busy having too much fun to pay attention to toileting signals so you need to teach him to get in the habit of paying attention and taking time out of his day regularly to go.
"It's time to go sit on the toilet. Your book/Blue's Clues/truck/coloring will still be here when you're done." Every half hour to start would be good. When he has an accident, you can start teaching him to clean himself. A wet pair of underwear doesn't feel good, but make him change himself every time. If he poops, make him rinse off in the shower and change into fresh clothes. It will be a matter of making it a habit and him learning the signals, and realizing that using the toilet appropriately takes less time than all of the clean up AND It feels better.
Praise him when he succeeds, but not over the top. Don't punish him for not succeeding, just make it business. Stock up on underwear and expect a lot of accidents in the beginning. Toilet training starts out messy, so you just have to commit to it.