Can things in your teens lead to later sleep disorders? My son stayed up late online through jr and sr years of HS and still sleeps 3 to 10.

Many patients. With delayed sleep phase disorder report that their problems began just the way you describe. Make an appointment with a sleep medicine specialist.
Habits hard to chg. Your son likely continues to do this because he can. Like a shift worker you can train your system to be different from "normal". The best predictor of when/how you tire & sleep is what do the night before.See a sleep doc if you wish, but until he has a motivation to change he is likely to stay where he is.