could it be that the doctors are wrong.There are other things beside add or adhd and autism.
Take and look up these other two and see if any of them make scene to you or if you think you should have your child checked for them also.
landau kleffner syndrome
Lennox-Gastaut Syndrome
I am sure there are a lot of children and adults that are not being treated for the right things.
I wouldn't put him on any of the drugs they use for adhd a lot of those have bad side effects.And they are what people use, as street drugs to get a high and very hard to come down from.
try Vitamins b6 and omega 3-6-6 and see if that helps .
My daughter has a dual diagnoses as well. I personally think that ritalin is a preference. I believe that diet can help a lot with the symptoms and characteristics of both diagnoses. I recommend looking into all forms of therapy and not just a synthetic med a doctor prescribes.