Hi , we've had this problem with our son since birth. He had some bad bowel problems and then just stopped going. He would hold it in for up to 2 weeks or more at a time , we had a really hard time getting him to go at all. One of the things we used , after trying every natural method of prunes apple juice extra fiber ect.., was a stool softener, this is not a laxative, it is not addictive and does not  make them become dependant on it as laxitives and suppositorys do. We used on called Lansoyl, its a Jelly , he eats 1 tsp/tbsp per day and this softens their poo enough for them to go without it hurting. Once he has a normal poo routine going , start pulling back the amount of softener your giving him until he is free and clear on his own. Hope he has some relief soon.

We had the same trouble with our boys especially the eldest. He would hold on until he was very uncomfortable all because his stools were too hard.

We got him to start eating more fruit and tried to soften things up a bit and once he had a few good experiences the troubles were soon forgotten.

We had to reward for going to the toot and play with his diet but all is good now.

Hope this helps you a bit .