My son is 3 and has Down Syndrome. From a tax point of view, is he disabled? He is too young to claim social security, or work obviously, so I'm confused.

At his age the question is moot since it won’t affect your return. You could answer it either way. 

No, not for tax purposes.   The question would only come in to play if your son was over age 19.   It won't make a difference since he is only 3.   See the requirements below for claiming a child over 19.  

Qualifying child

• They're related to you.
• They aren't claimed as a dependent by someone else.
• They're a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They're under the age of 19 (or 24 for full-time students).
  • No age limit for permanently and totally disabled children.
• They lived with you for more than half the year (exceptions apply).
• They didn't provide more than half of their own support for the year.