HUGS! How long ago has his nap gone? It could still need some sort of nap every few days, and not having it he became OT. Anyway, since nap dropping at 3.5 yo, my DS needed 12.5-13 h A time in the day to have a good night and his night sleep consistently is of 11-11.5 h (it was more 11.5 h just after nap dropping, more toward 11 h now), so it could be that 11 h nights and 13 h days are what he needs. In this case, you would need to put him to bed later, to have a later wu.
How is his behavior when he wakes at 6 with a 7 pm bt?