Autistic kids are deficient, many times in fatty acids, B vits, enzymes & minerals. These needs must be met and maintained for true healing to begin. All the cleansing and chelation will only further debilitate a deficient body.
I would suggest seeking out a molecular nutrition specialist, who can determine a proper nutrient balance for your boy. Get the bowel issue corrected, strengthen the body, then get him on Miracle-Mineral-Supplement and I would also suggest following EM's advice on biofeedback, it is excellent for brain wave balancing.
To remain on parasite cleansers for a year and a half is irresponsible on the part of those doctors that you a seeing. I would get a second opinion.