If he was not a full-time student for part of 2021 then you can only claim him as a qualified relative if he earned less than $4300 in 2021. That sounds unlikely if he worked full-time for six months and then joined the military.
IRS interview to help determine who can be claimed:
https://www.irs.gov/help/ita/who-can-i-claim-as-a-dependent
https://ttlc.intuit.com/questions/3113432-who-can-i-claim-as-my-dependent
**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.**
