For income tax, "disabled" means unable to perform gainful work (work for money). If your son can work a minimum wage job, he is not disabled for tax purposes, even though he may have a medical diagnosis. Because of the focus on gainful work, many people with medical disabilities are not disabled for tax purposes.
Then, even if your son is disabled, it doesn't do anything for your taxes unless he is over 13 and you still need to pay for day care to watch him (because he is unable to care for himself), or he is over 19 and you need to claim him as a dependent.
