Potty Training for Keegan

Ahh! This has been more difficult than I thought it would be. He has had dry diapers when he wakes up in the morning for about a month or so now, so we knew it was time to start the potty training. We have started putting regular underwear on him to make him more aware of when he goes to the bathroom, and whenever he's wearing the undies and needs to go poo-poo, he ALWAYS goes on the potty, which has been amazing. It's getting him to pee on the potty that has been difficult, he will start to pee in his undies then yell "I need to go on the potty!" but by then it's too late. Any tips?? He is already registered for school starting this fall but he can't go if he's not potty trained, so my deadline is coming up in about four months. I want to get this ball rolling, lol!