My older son age 25, has been diagnosed with clinical depression 4 years ago. He dropped out of school and leaves home. He held some jobs, but lost them. Is he disabled?

For IRS purposes, disabled means unable to perform gainful work (working for money) due to a condition that is permanent, or expected to last at least one year, or expected to lead to death.  Because the IRS focus is on gainful employment, people who have medically recognized illnesses or disabilities may not be considered disabled for tax purposes.  Only your son's treatment providers could certify that he is unable to perform gainful work due to a permanent condition.