3 year old pooping constantly

Hi. I have a little boy who is nearly 3. We started to potty train him a month ago and he was delighted. He was having ones and twos on the potty within a day and was very pleased with himself. He has always had regular bowel movements and has had very good control for 2 years, he always filled his nappy in private in the corner of the hall. However the last 10 tens have been awful. He has started to poop constantly throughout the day but wont do it on the potty. He isnt having large poops, its just 'seeping' from his bum all the time. It is really soft, not hard at all. He wont do a large poop. He is still happily weeing on the potty and does sit on the potty to try and poo, but its like he cant manage to get a substantial one out.
It really upsets him when he can feel the poo coming out and he knows he should sit on the potty but he does a little pain dance. We have tried everything. He has a very good high fibre diet and I cant say he's constipated because if we added all the wipes up (a full packet!) that we use in a day it would be a large poo.
I am really lost and dont know what to do, he doesnt want to go back into nappies or pull ups, but we cant carry on like this. If we leave him with no pants on we have to wipe him every 2 minutes, but if we put pants on we would go through 20 pairs a day. Please help.