There were three stimulus payments. The first two, $1200 and $600, are technically based on the person's 2020 dependent status. If you son was your dependent on your 2020 return, and did not already receive one or both of those, he will not receive it now. You do not get it, instead as those payment did not apply to dependents (there were $600 and $500 payments for dependents under 17 only).
The third stimulus, $1400, was sent based on dependent status on the 2019 tax returns. If you claimed him for 2019, you should have gotten it. If he claimed himself, he should have gotten it. If neither of you got it, you may have to wait until you/he file your 2021 tax returns in 2022. Technically, the third payment is based on 2021 status.
See discussion at
https://ttlc.intuit.com/community/tax-credits-deductions/discussion/3rd-stimulus-dependent-eligibili...
Also see:
https://www.irs.gov/coronavirus/second-eip-faqs
https://www.irs.gov/coronavirus/get-my-payment
