You can claim him as a dependent under the Qualifying Relative rules if he meets all the requirements. If you do claim him you will be eligible for the $500 Other Dependent credit.
To be a Qualifying Relative -
1. The person cannot be your qualifying child or the qualifying child of any other taxpayer. A child is not the qualifying child of any other taxpayer if the child's parent (or any other person for whom the child is defined as a qualifying child) is not required to file an income tax return or files an income tax return only to get a refund on income tax withheld.
2. The person either (a) must be related to you or (b) must live with you all year as a member of your household.
3. The person's gross income for the year must be less than $4,700 (social security does not count) in 2023
4. You must provide more than half of the person's total support for the year.
5. The person must be a U.S. citizen or a U.S., Canada, or Mexico resident for some part of the year.
6. The person must not file a joint return with their spouse.
it doesn't seem like he attends school for at least 5 months of the year. therefore, he can't be a qualifying child - dependent. however, he can be a qualifying relative - dependent if
Qualifying Relative and eligible to be claimed as a dependent the rules are:
1) gross income for 2023 less than $4,700
3) you provide over ½ his support
4) isn’t a qualifying child of another taxpayer