Up until this past weekend my 28 month old was sleeping through the night (been in big boy bed since Christmas). His routine has not changed, he takes a 2-2.5 hour nap daily, bedtime has not changed, the only thing that has changed is that he would typically sleep until 8-9 in the morning, but for the past 2 weeks he's been waking up at 7-7:30. I thought he was just overtired, but in the past that has usually resolved itself within a couple days, not this time. I'm worried he's getting bad habits.
It is best to stick with what you have always done. Often something simple, like the change to daylight savings time or being ill, will cause sleep to go awry. Sticking to what has always worked is best. You can also try adding a “good morning light” – a nightlight on a timer that is set for usual wake time. Let him know that when his “good morning light” turns on that is when he can get up.