hey hun well we havent really had the same problem as u but i suppose something similar. charlie is now 3 1/2 and has been potty trained for over a year....understanding that having a poo on the toilet/potty was just as good as doin a wee took a few extra weeks to grasp but we got there in the end....but basically a few months back charlie seemed to go backwards and wouldnt go for a poo on the toilet and would simply just do it in his pants.... this completely threw me as nothing had changed within our family or routine etc in order to affect him in any way so i was a bit shocked lol anyways when he first started doin it i would not make a fuss just say that he needed to do it in the toilet and clean him up and put new pants on etc, this seemed to have no effect so i then showed him i was cross and told him that it was not acceptable and that he needed to start doin it back in the toilet as he was now a big boy etc....this seemed to work for a little while and when he looked as tho he was goin to fall back into pooing in his pants i decided to do a reward chart with 4 different things for him to get stickers 4 but one of them was to keep his pants clean and this seemed to work and he went back to goin on the toilet...there are still days where i can tell he desperatly needs a poo and is avoiding goin on the loo so i just get him there as quick as i can without too much fuss.
i dont know if any of this will help but also just thought has ur LO got enough fibre i his diet to help him to go???? also u could try and explain to him that the longer he leaves it the more his tummy will hurt and the harder it will be to go etc.
i'll keep my fingers crossed for u hun good luck
take care x