Professional help?

One of my children is heavily autistic and nonverbal at 6. He had constipation issues before which made him fearful of going #2. He trained himself to just never have a bowel movement, so he soils his diapers all day (his bowls are full and it's the natural progression out). It concerns me because he's continuously backed up and that's not good for his long term health.

Doctors have been little help. The gastro put him in the hospital with a drip to essentially clear him out completely but a week after we were back in the same boat. Our pediatrician has us giving him laxatives daily, and even though we do, this doesn't change the situation. The ABA school he goes to takes him to the bathroom very routinely but this also seems to be ineffective. I've tried putting him on the toilet after meals (he's cooperative to sit there), but he seems to still hold back.

Is there some place we can go to for help?