Just because you cannot claim your son as a dependent under the Qualifying Child rules does not necessarily mean that you can no longer claim him as a dependent.
You can claim someone as a dependent who is age 19 or older under the Qualifying Relative rules if they meet all the requirements under the rules. If your son had gross income in 2017 of less than $4,050 and you provided over one-half of his support then you can claim him as a dependent provided he also meets the rest of the rules.
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 $4,050 (social security does not count) in 2017
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.
