You might want to amend your return. Adding him as a dependent will give you agitational tax credit for other dependent and potentially could put your in a different tax bracket. The only concern, to process amended returns usually takes time, and the IRS could finish the third rounds of stimulus checks by the time they will process your amended tax return. If your ex-husband claimed him as a dependent in 2019, the IRS could use 2019 information and send the payment for your son to you ex-husband. You also can file your son return with 0 income and mark there he's a dependent. It is hard to tell if it helps, it could.
