J.M.
First of all, is he really not sleepy? If that's the case, you can try bumping back his bedtime. I find my daughter gets out of bed a lot if she's really not ready to sleep. Otherwise, here are a couple of suggestions:
1. Only tuck him in twice. Once do the regular bedtime routine, then when he gets out of bed, say "this is the last time I'm tucking you in - now you have to get back to bed yourself." Kind of miserable that night, but normally keeps my daughter from getting out of bed subsequent nights.
2. Say before you leave the room "okay, is there anything else you need or need to tell me?" Then they don't have the "oh, I forgot to tell you X" excuse.
3. It really helps my daughter if I give her something to think about while she falls asleep. Something detailed. I normally say something like "Imagine that you're a princess in a castle, and imagine all of the beautiful flowers in the garden, and all of the delicious desserts that you'll have at tea, and all of the beautiful dresses your princess friends will wear." It keeps the mind from wandering and helps pass the time until she drifts off (works for me too! Although I don't imagine being a princess, ha ha). You could do the same with a construction site or train scenario, if your son is into stuff like that.
4. Finally, I have some friends whose kids really need a little wind down time by themselves in bed before they fall asleep. They put the light on a timer so their kids know they can play a little while by themselves before they doze off. Then it doesn't matter so much if they are sleepy, because they're allowed to play a bit. Doesn't work for my kids, but might for yours.
Good luck!