Severe constipation in my 3 year old?

My son has had severe constipation since he turned 1 years old and I stopped feeding him. It gets so bad, he will shake, sweat and can not physically walk. Today, he crawled into his bed at 11 am and didn't get out once. That is not normal for a 3 year old obviously. I have been to the DR's for this multiple times, we had an x-ray done and it shows him being backed up in the DR's words, "pretty darn bad" all throughout his colon. We have done enema's, stool softeners, miralax and the DR's "10 day plan" He might or might not go when we do these things but it never completely clears him up and he always goes back to being constipated within a few days to the point his belly is swollen (you can physically see his belly de-swell when he poops) I am getting so frustrated with his DR because I don't feel he is taking it seriously enough, my son is in pain and I don't like that! My son has an extremely limited diet due to autism sensory issues and we are in food therapy with him trying to help that but that can take years and I need help now. What do we do? What else can I request the DR to test for to see if we can find out why this happens to him?