Some of the IQ and achievement tests in common use rely on social skills or social logic that a child with ASD simply doesn't have in place (yet).

Sometimes you can get a better measure on the Differential Abilities Scale or the Raven. But I'd say it's probably not urgent at this point to have a good number-- better to find a clinician who understands the complexity of him beyond his test scores, and is willing to meet him where he is in working on the missing skills.