You can mark the disabled box for him, however it will not have any affect on your tax return at this time since the child is younger than 19.
Indicating a dependent as disabled is only effective when the dependent is your child and is age 19 or older so the Qualifying Child rules will be in effect instead of the Qualifying Relative rules for claiming a dependent. This means the child's income is not a factor for claiming them as a dependent and the child can be part of your Earned Income Tax Credit eligibility.
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