We are seeing a DAN! doctor. My son is not technically autistic, but he has some quirks and has responded very well to the "autism diet," so we're trying to see what other things are available to help us. Anyway, when things got very bad constipation-wise for Tom, I called our DAN! doc. He had us use magnesium citrate (for Tom, it was 6 oz because he was 6 years old. For your son, maybe 4 oz because he's 4?). You also give the same amount of water. Once it's all gone, you wait 1/2 hour and then give an enema. Mag citrate brings liquid to the colon and it will help dissolve the stool in there. Then, the enema comes from the other end to encourage everything to flush out. We were told to repeat this as many times as needed until all we got was liquid stool. That first time, we did it 4 times until things were "clear." (We actually got liquid stool the first time, but I knew he was still backed up, so I repeated it until the blockage came out and everything was clear after that.)

Once you get him clear, you need to track things. Do not let him get backed up so much. Part of what happens is that it hurts to go, they try to avoid it, the colon gets bigger and bigger, because the colon is bigger they don't feel the urge as often, it hurts to go... a bad cycle. Anyway, we were told to repeat the mag citrate / enema at the first sign of trouble. Don't wait for it to get bad. It takes time for the colon to shrink to a more normal size.

A few other things...
*We found that some of the artificial sweeteners (manitol, sorbitol) that were in some of his supplements contributed to the constipation.
*Fruit (and fruit juice) does not help Tom's constipation. While it does seem to soften the stool, it also feeds the yeast in his intestinal tract, and the yeast make it harder to push the stool out. Tom seems to do better the less fruit he consumes.

Last, I'm not sure what kind of diet you're feeding him, but dairy is known to be constipating. We've gone pretty Paleo (caveman) in our diet, and Tom is doing much better, in more ways than poop.

Good luck.