Potty Training

My son will be 5 in 2 weeks from now. He has been potty trained since he was 3. But for the last 5-6 months he has been pooping in his pants. At times its almost every day, and then he will go in the washroom 4-5 days in a row and then back in his pants. It is becoming very frustrating! I ask him why hes going in his pants and he says "I dont know, I forget" So then I remind him. When he does go in the washroom I congratulate him but then once again he goes in his pants and doesnt tell anyone until I find some on the floor or ask him to go to the washroom and he says "actually I went in my pants" and its dried to him and all down his leg. At first I was angry and punished him, but then I found that didn't work and wasnt very good so then I started to support him with gentle reminders and it still isnt seeming to work. I just dont understand! Please help!