My son doesnt ask when he needs a poo!!

My son is 3 yrs old and has been dry without any problems for about 4 months, but he doesnt ask when he needs a poo, he goes into a different room and poos in his pants!! If I catch him about to do it I take him to the bathroom and he uses the toilet and is very pleased with himself. I have tried everything rewards, tons of praise, I dont get cross with him when he does it in his pants etc.......... Hes not scared of the toilet as he wees in it fine, and I think he must know when he needs a poo as he leaves the room. Any advice would be great as im stuck on what to do...HELP!!!!!!!!!!!!!!!!!