We do not know why. If your son was under the age of 17 at the end of 2020 and he has a Social Security number then you can get it on your 2020 tax return by using the recovery rebate credit.
The stimulus check is an advance on a credit you can receive on your 2020 tax return. If something went wrong or you did not get the stimulus check this year, you can get it when you file your 2020 return in early 2021—if you are eligible.It will end up on line 30 of your 2020 Form 1040.
https://www.irs.gov/newsroom/economic-impact-payment-information-center-topic-j-reconciling-on-your-...
https://www.irs.gov/coronavirus/coronavirus-tax-relief-and-economic-impact-payments-for-individuals-...
https://ttlc.intuit.com/community/tax-topics/help/how-will-the-stimulus-package-impact-me/00/1393859
good post with screen shots for recovery rebate credit
https://ttlc.intuit.com/community/taxes/discussion/filed-as-an-independent-and-single-but-haven-t-re...
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
