J.M.
Hi A..
I agree with Liz. Rather than asking him why he did something, a more fruitful conversation would be: "I see that you're not sleeping. Maybe you feel like playing but its time to go to sleep." Basically its observing what he's doing (and relying it back to him. Acknowledging his feelings/impulse and then redirecting it back to him). Its true that children dont fully understand why they do some things. You probably know why he's not in bed more than he does!
As for the constant string of why questions, I agree with what others are saying. Its a way for children to communicate and make sense of the world around them. My son will ask me the same question over and over until he has internalized the answer. On some days, this take an enormous amount of patience!
Good luck.
J