Ds2 was 12 when he had to have movical for impactation, four sachets day one, 6 day two, 8 day three, 10 day four, 12 day 5, before day 6 he would have to go in a hurry but nothing to write home about, on day 6 the explosion arrived, it was volcanic but upside down if you know what I mean, he felt no pain at all, we basically just camped beside the loo with books, DVD player and iPad tablet thing he had at the time, he was very tired and slept on and off but after 48 hours he was clear, and we were free to move outside 
It still happens but nothing as bad as before and usually a couple of sachets will shift it, oh and leboob is right, cut out the milk, that was the first thing doc said to me, lay off the dairy, he has autism and dairy products was one thing he tolerate but hey ho, the pros outweighed the cons.
Anyway sorry for essay style answer and I hope your little chap is doing better, if he can get him to drink plenty of water, take care.
RomComPhooey, hope,your chap is ok, that's so tough for him.