This seems to be a common problem but the children tend to be a lot younger. DS1 3.9 has been dry in the day for about a year but will only poo when we put his night time nappy on despite promises of a present or the bribe of a chocolate button if he will go in the loo. We offer him the ipad to watch a favourite TV show, giving privacy, the lot. He never soils himself, always holds it till the nappy goes on. He gets really upset when we push it (no pun intended!) so we have just left it for now.
He doesn't appear to be in any pain pooing and he has, in the very early stages of training used the loo or the potty to poo. We encourage him to use the loo to wee as I think it's more appropriate given his age. We have a child loo seat and a potty upstairs and would be more than happy to let him use the potty for pooing!
I don't know whether I should take him to the GP and get movicol which seems to be highly recommended on here or whether he will just sort it out himself. Please reassure me we are not alone!