Alfie Kohn's book, "Unconditional Parenting" talks about the pitfalls of using rewards and punishments to motivate a child. (He sites research which shows that kids might change a behavior for a while, with one or the other, but empirical evidence states that these changes are rarely long-lasting positives.)
All that to say, your son may be ahead of the pack in that particular regard. Now, the key is to find out what helps him self-motivate.
If it were me, I'd spend time in observation, and then ask gentle, open-ended questions. He's written one line on each page. "Tell me about those lines that you drew. (Pause--) I noticed that you put one on each page. Tell me about that."
"I saw you really liked playing (cars, dinosaurs, blocks). What was happening when you were playing ________."
"If you could do anything you like tomorrow, what would you choose?"
Keep the questions conversational and simple. One strict warning here: Do not ask "WHY", because this will shut children up like a clam. They cannot answer that sort of "why" question. (Bruno Bettleheim has much to say on this in "A Good Enough Parent".)
Parents have to be detectives, and detectives start first by observing. That's where I'd start. See if you can enter his world a bit and find out what excites him.
Another thing that comes to mind: Do you think he wants company for some of what he's doing? I've found that when I send my four year old in to get dressed, he often finds something else he wants to be doing. If I go in with him, spend some time talking with him and keeping him on task, things go more smoothly. It's my company he's responding to, taking my lead. My guess is that when we give them opportunities to be self-directed (esp. after a day at preschool, when so much is organized for them), they take us up on it by not attending to the tasks we've assigned. So being available for some of those tasks--not to do them for him, but just to be company-- might be worth a shot, too.