Regression is somewhat normal for children. I'm curious at what age his training starting, regression is normal within 6 months or so afterwards. It seems that there is a reason, sometimes especially with a baby in diapers, he may feel neglected of attention, or so. He may not want to start school and be away from mom, and knows by hearing someone say, if youre not p.t. then you cant start school.
I would definately do more research on this, but the advice I offer is do not shame him and definately reward his efforts of big boy pottying. Stickers or something that he enjoys would be of help. I let my little boy put stickers on the dryer (clothes dryer) that way he could see his acheivement. There are many different stragties that work depending on his personality and his interests. Good Luck.