My son is in the autism spectrum. He as a medical diagnosis. Should I answer "yes" to the question that asks if he is disabled?

The question does not affect your return if the child is not of employment age.  You can answer "no" since it does not really apply.  The definition by the IRS is: You have a permanent and total disability if you cannot engage in any substantial gainful activity because of your physical or mental condition. A physician must certify that the condition has lasted or can be expected to last continuously for 12 months or more, or that the condition can be expected to result in death.