I don't have kids, but I have always had trouble sleeping, since I was a baby, according to my parents.
For me, the problem was not that I couldn't go to bed early, it was that I woke up way too early in the morning, and also in the middle of the night. I frequently found myself wandering the hallways in the dark because I was bored and couldn't sleep.
My parents' solution was just to make me stay in bed in the morning until 6 am at the earliest. That didn't really help me sleep, though .
I suggest you try to start a routine for your son when he goes to bed, if you don't already have one. That helps a lot of people get into "sleep mode." I know I have a hard time winding down sometimes, my mind always seems to be racing, and it may be the same for your son. Try setting the time a little earlier than his usual bed time if he seems to need more sleep. Give him a glass of water, and maybe either read to him, or have him do some reading in bed, before he goes to sleep. You probably already know this, but definitely avoid giving him any sugary foods a few hours before bedtime.
Sensory issues may also play a part in keeping him awake. Make sure he's comfortable with the temperature in the room, and that the bed is comfortable, as well. Pay attention to the amount of light he likes, and if necessary, get him some earplugs. I have thick drapes I made for my windows here at school because we get a lot of street light even at night, and I wear earplugs, and these things help me out.