R.,
My son is right there with yours. He has been going through it since he was 8 months old and now he is almost 9. We are trying to weane him off the miralax. However that is a slow process in our case due to he fact that our son is an extremely picky eater. But you do need to stop the dairy, that is a big culprit. You do need to get fiber into his diet and get him into see a gastroenterologist right away. When my son gets to bad I have to give him pediatric enemas at home to help relieve the constipation. I don't recommend this unless it is a last ditch effort.
I wish you all the best, however I do recommend having him looked at by a specialist they will be able to give you a better idea of what is happening with your little guy!
Good luck from another mother in the same boat!
S.