You would have to amend your return to make the change. If you amend, amended returns must be mailed. At this time the IRS is not processing mailed in returns. They won't process mailed in returns until they re-open, so your amended return would not be processed in time to receive the stimulus money.
The stimulus money is actually an advance on your 2020 return (filed in 2021), so if he qualifies, he could receive it if he files a 2020 return.
