What happens when he needs to do a poo? Does he let you know/request a nappy or does he just go in his undies?
Is he fairly consistent in the timing of his poos?
My DD was TT at 3 for wee but wouldn't poo on the toilet. She started preschool at 3.5 despite still not using the toilet for poo. But she only ever did a poo at night after her bath and would ask for the nappy to do it in.
I alerted preschool to the issue and send a pull up in her bag just in case but she never needed it. About a month or so later she finally got over the fear and all was fine. I don't think she ever did a poo at preschool though - she prefers our toilet