I would suggest that you send him to school ready to learn (well rested, well fed, as calm a home environment as you can reasonably manage), and then let the teachers do the rest. He does not have homework yet (at least I hope not in Pre-K!), so when he gets home, have fun with him! He's at an age when he needs to run, jump, climb the walls (okay, maybe not!), and be a kid. I would rebel if I came home from work and had to do flashcards, wouldn't you?
I have taken a mostly hands-off approach with my kids' education. That's NOT to say I'm not involved - I'm talking to the teachers every day, making sure homework is getting done (my kids are in 1st and 3rd grades), but I figured that what happens once my kids walk through the classroom door in the morning until they walk back out in the afternoon, is up to their teachers. I can't micromanage that process. The teacher needs to build up a rapport with my child and she needs to find a way to reach my child using her own strengths as a teacher. If I jump into the middle of it, it's going to undermine her as a teacher. So far this strategy has worked for us.
So I'd recommend just backing off. Be his mom, not his teacher. Find fun things to do together - all kinds of experiences can be educational. He will find the motivation within himself once he's not under pressure to perform. You are a good, concerned mom, wanting him to do his best! Don't worry, it will happen!