My son will only tell me AFTER he's pooped.

My son will be 3 this summer and he will sit on the toilet but he won't go. Then he will go right after he gets off the toilet and come cry and want me to change his diaper. He pooped on the toilet once but he was half way done pooping and I picked him up and put him on the toilet. He knows what it is and says 'toilet' and 'potty' but still poops his diaper and cries after, I think he knows he was supposed to use the toilet and didn't. I try not to force him or get mad when he doesn't use the toilet but also encourage him to and tell him big boys use the big boy potty but he still just poops after he gets off the toilet... What else can I do?