Bowl Trouble with a four year old

Patient: I have a 4 year old boy, he will be 5 in September. He has been potty trained as far as going pee in the toilet for about a year and a half. how everwe are unable to get him to go poo in the toilet. He says that he does not know when he needs to go. When he does mess his pants it is only poo and it is not wet from pee. Which leads me to beleive he is not straining or pushing using his muscles to go. Is it possible that he could have a spastic bowl or something that is causing him to really not know when he needs to go poo?