Can you claim your 27 year old disabled son who is on ssi?

Without knowing the full details of the situation, it's impossible for us to give a definitive answer. There's a flowchart/worksheet in the Federal tax instructions that you can complete to determine whether or not you can claim a given person as a dependent. If you can't figure it out on your own, hiring a tax professional to prepare your taxes for you is much cheaper than guessing wrong.