Not potty training related as such, as my son has been "dry" almost a year now. I'll give as much background as I can.
My son (who turns 4 in a couple of months) was terrified of the potty/toilet when he was 2ish and so we didn't push the issue, we waited until he was showing less resistance. About a month before he turned 3 we cracked it and he only took a couple of days to be "dry" but he seemed terrified to have a poo. We had a few poo accidents in the first couple of weeks, as he seemed to be holding it in for that long that he was getting bits in his underwear (sorry).
He seemed to relax about it all after starting nursery a couple of months later and we had no problems with him going when he needed to go.
However, fast forward 6 months and we are back to holding poo in for as long as humanly possible, being visibly uncomfortable, getting marks in his underwear from holding it for so long and then obviously being in discomfort when actually going because of the sheer volume after holding it in. If you ask him if he needs to go he will get very upset and say he doesn't when he clearly does.
I have no idea where to go from here as he starts school in September and I don't want him having poo accidents purely because he is afraid to go for one! We have tried not mentioning it (he will hold it in and hold it in until he has an accident when walking or in the bath), we've tried asking him to sit on the toilet for a poo when we think he needs one (this leads to hysterical crying and screaming that he doesn't need to go, usually resulting in a poo after 15 minutes of this) and we have tried letting him go on the potty instead of the toilet (with a mixture of the above results).
Any advice greatly received as I am trying my best not to turn it into a battle, but he is just getting worse with it instead of better.