Please Help!!!

My son has been in underwear since June. He still doesn't tell us when he has to go. He has also been having accidents at PreSchool. He was doing well at school, but recently he has started pulling down his pants and peeing on the floor in the bathroom. And, of course, all I get is "I don't know" when I ask him why he didn't tell us he had to go to the bathroom or why he peed on the floor. I just don't know what to do anymore. Any advice would be greatly appreciated.