We had this problem with my eldest... unfortunately a problem that I thought would be a few weeks to fix turned into a problem that took years to fix. The doctor pretty much told us that upfront, but of course, I was hoping he was wrong. Having said that, I hope yours is a temporary issue! I would definitely recommend Miralax. We mixed it in my son's milk, and he didn't notice it at all. Our problem became that he was afraid to go because he associated going with pain, and so it was a vicious cycle. We even took him to a gastroenterologist because I really felt it was becoming a behavioral issue, and my pediatrician didn't agree. The GI specialist did agree and couldn't find any physical issues that needed to be dealt with. We did have to do some enemas and finally got it under control with Miralax on a regular basis. It was a long road, and I had no idea constipation could wreck such havoc! I've hoped and prayed that my two younger kids wouldn't have the same issue, and so far we've been good
I hope this gets resolved for your son soon!