Thanks for your responses.
@SweetieJean He is receiving survivor benefits, so he is eligible to receive it (https://www.irs.gov/newsroom/who-can-get-more-economic-impact-payment-money-for-children).
@KrisD15 The child (just turned 18) doesn't have a dependent, he is one for the mom. Both child and mother received a letter saying, "Under the American Rescue Plan, the IRS issued you 2021 Economic Impact Payments for the following total amount." Just the mom's total is $2,800 while the child's reads $1,400. Upon further inspection, the child's letter is addressed to "<mom's name> FOR <child's name>" so from your message I guess she would have got it for her 17-year-old son anyway.
The child doesn't have an "IRS Your Online Account," but from what you say, if he creates one, he'll be able to see there if any checks were sent to him.
