If he he had gross income of more than $4300, then he would not be considered a dependent. Gross income includes all income not exempt for tax. So if he is receiving social security disability only, that would not be considered taxable income and you could claim his as a dependent if he meets the other thresholds for a qualifying relative (Support test). Your brother could still fill his own tax return, if you did not claim him as a dependent to claim the $1400 stimulus.
We don't have specific information on the income qualifications or procedures for the third stimulus payment because the measure just passed Congress. If you had issues receiving your earlier stimulus, then the best way to ensure that you are one of the first to receive any future stimulus payments is to file your 2020 tax return as early as possible and use direct deposit so that the IRS will have your updated information.
You can find the most up to date information from the IRS here, and updates from TurboTax here as they develop.
