There are two basic dependency rules. The "qualifying child" rule and the "qualifying relative dependent" rule. It sounds like your son qualifies under the qualifying relative rule.
- He earned less than $4,300 in 2020
- You provided more than 50% of his support for the entire tax year.
If your son was a student in 2020 then I do question the support. So you might want to look at that. Especially if they received scholarships, grants, 529 funds in 2020. (probably not if the fall semester of 2020 was the final semester of their senior year in college.)