Hi and Welcome!
Well, I'm sure you've already looked at whether he is getting too much sleep during the day? And that he is getting plenty of exercise out doors in the afternoon? When our DD was younger we had firm rules about getting up that flexed with age. At 6 we were (if I remember correctly) letting her read quietly or silently play with her paper crafts. If she needed something that required an adult, she could call us from her bed. This was only allowed in the morning; at night she just had to hang out with the light out (no book, no stuffed animals, nothing at all). We all discussed it and talked about the rules and the reasons for the rules. She was free to come out as soon as one of us came and said "Good Morning". Since my DH is a crazy early riser, she did not usually have to wait.
If all else has failed, I would consider talking to his pediatrician about a consult with a sleep specialist. If he is going to bed early and waking early, they may have you use a light box to shift his circadian rhythm.
Hopefully, you'll get more suggestions from people who have BTDT
