You may just have a 1.5 hour nap. The nap might get longer later, but not sure if tweaking will work. Certainly if preschool is going to move the nap to 12:30 you might want to start practicing that time zone so that he's not totally surprised by it when school starts. Preschool can be pretty exhausting at this age, so having a new nap time AND preschool might really be a double whammy for him.
Even if 1.5 hours is all he'll do right now, it doesn't mean that it will only get shorter from here. My son started doing 2.5 and even 3 hour naps after he hit age 3. So, if changing nap time doesn't make nap longer, don't despair! Life events could conspire to make him sleep longer at a later date.