I did the "cold-turkey" method with my daughter at 3 years 7 months and she did great. She was in preschool also and even though they had to be potty trained, the teacher let it slide because she would have rather have had the kids in there wearing a pull-up then having to miss out because they were not trained. Classes were Tues and Thurs so we picked a Fri (giving us a good 4 days) to start potty training. By the second day DD was trying not to wet her pants and was letting us know she had to go so we could get her to sit on the potty. We did put a pull-up on her for bedtime because we couldn't be sure how she would do and we did send her to school in a pull-up just in case, but she did fine. Within a couple of weeks DD was asking to go to school without the pull-up!
You can start with sending him in the pull-up this week, but if you find that he keeps wetting in them, start sending him in the underpants and provide a change of clothes. Have him go potty at home right before you leave and don't let him load up on liquids in the morning. Hopefully when he is at school if he has to pee he'll remember to go to the potty, or he just won't have to go as much until he gets home. Ideally you could work more on potty training when he has more days off from school.