My DS did great with lights on a timer rather than gro-clock, same principle though, he was younger an still understood it but I would not have expected him to wait from 5/5.30 to 7.30/8am. With such a big change in the WU time I'd be looking for other reasons.
Is he waking due to baby having a night feed?
Is his room dark enough (I had to re-black out DS's room even at 5yo for summer, he already had black out curtains but it was not enough for the summer sun in the early hours, the only way to keep him asleep was TOTAL black out)?
Is he night time dry or still in a nappy? Is there a chance he might be getting up because he needs to wee?
Have you asked why his light is on in the night? What does he say?
Is he getting enough physical exercise in the day? Mine has phases of needed LOTS of exercise otherwise he is UT for the night sleep, he doesn't resist BT but rather he wakes in the night and finds it hard to get back to sleep. With extra exercise he sleeps better at night.
Is he near birthday or half birthday? Those can effect sleep.
If yours is waking and just doesn't know if it is morning or not then the gro clock or timer lights can be helpful as a sign to him to just go back to sleep (if he can, if all he needs is telling it is night time and he just settles back down and goes to sleep alone) but if he finds it hard to get back to sleep then something additional is needed yk?