3.5 son waking up at 5am consistently. Looking to improve his wakeup time. Any suggestions?

So our son goes to sleep normally around 7 to 730. 8pm at the absolute latest. He goes to preschool and we do activities with him in the afternoon so he is tired by the time we go lay down. We normally have no issues whatsoever with bedtime. That being said, he is waking up at 5am consistently. We have a sleep clock in his room that stays red until 615am and he knows he has to stay in his room till it turns green. This has been happening for a couple weeks now. We are just hoping for maybe 6am. Any suggestions are appreciated!