Poopski!!

So my 3 and a half yr old son has decided after a year of being potty trained to start pooping in his pants. Hes been doing it on and off for a month now. We have tried time outs, taking toys away, putting diapers on him to make him see that its what babys do. He is still doing it. I read that it can be out of laziness, but he goes pee everytime just fine. It could be for attention. Hes always got that!! So I don't know what to do. Hes not pooping alot. Just small amounts at a time about 4-5 times a day. So yesterday I gave him a supository and about 45 mins later he had a huge poop. So far today he hasnt pooped at all. So I guess I will see if he starts doing the same thing again. If so I am going to take him to the doctor becuase maybe its a bowl problem. If anyone has any ideas or has been threw this I would love to hear some solutions..