"Full time student" means attended full time for at least 1 day in 5 or more months. So a student who graduates high school in May would be a full time student for that year. If your son only attended school 3 months, that does not meet the qualification for "qualifying child" dependent.
For qualifying relative dependent, you have to provide more than half his support AND his taxable income must be less than $4300.
If he attends full time for part of 5 or more months in 2022, he would qualify for 2022 even though he does not qualify for 2021.
