My son had chronic constipation which miralax did not help. Through research I discovered Dr. Janet Lintala, who wrote a book called "The Un-Prescription for Autism: A Natural Approach for a Calmer, Happier, and More Focused Child". It focuses on using digestive enzymes. The book contains a protocol which includes a type of probiotic called Saccharomyces Boulardii, which is commonly used to treat and prevent diarrhea. I've been following the protocol for a little over two months and have noticed improvements in my son's constipation and behavior.

I also recommend reading "Enzymes for Autism and Other Neurological Conditions" by Karen DeFelice.

Hope this will help your son.