You can claim your child as a "qualifying child" dependent if they live in your home and is permanently disabled. Disabled for tax purposes means "can't perform substantial gainful work." Gainful work is employment for money. Working in a sheltered workshop as occupational therapy is not considered gainful work even if he is paid, but working a minimum wage job at least part time will usually be considered the ability to perform gainful work.
You can claim your child as a "Qualifying relative" dependent if you pay more than half their support and they have less than $4500 of taxable income. Taxable income does not include social security but could include investments in the child's name. The child does not have to be disabled or live with you to be a "qualifying relative" dependent.
Each type of dependent is worth a $500 tax credit. If your child lives in your home, and is disabled, and if you pay for care (day care) to watch your child so that you (and your spouse if married) can work, you can also claim the child and dependent care credit.
You can file amended returns for a refund for up to 3 years after the filing deadline. Since the deadline for the 2019 tax season was extended to July 15, 2020, you have about 18 days to file an amended return for the 2019 tax season. You have plenty of time to file amended returns for the 2020, 2021 and 2022 seasons.
