Have him try every 20 or 30 minutes at first, then stretch the time as you learn his patterns. Don't ask, he'll say he doesn't have to go, but tell him it's time to potty and take him and sit him down.
Pullups are diapers so get rid of them, they keep kids dry so of course they want to keep going in them, then you can clean them up! Put him in cloth training pants all the time, day and night, so he knows there's no going back. Once he gets good at going regularly in the potty or on the toilet you can put him in regular little boy underwear.
Don't use diapers when you go out. You can keep a small potty in the back of your car and pull over for him to use it when he needs it.
Praise and encourage him for successes and trying. Have him help clean up his accidents.
Drive by the school and show it to him, get him excited about going. Remind him he gets to go when he uses the potty all the time.
Above all, be consistent. You can't be lax one day and an enforcer the next. Tell him what you expect from him and stick to it ; )