Thank you for writing back to clarify your question. Here is how it works:
First of all, here is the new credit under TCJA Credit for Other Dependents.
With the TCJA, the personal (dependent) exemption is out. They invented this "$500 Credit for Other Dependents" which basically is the same rule as the old one for claiming a dependent. But the dollar amount is different. Instead of $4,150 per person, it is reduced to $500. Under the "Credit for Other Dependents" rule, you would first look at the Qualifying Child, which your son fits in because he is a full-time college student and under age 24. I point out the importance of the support test because he does have an income of 15,000. Even you claim he meets the support test, IRS might question how he passes the test with a considerable amount of income for himself. To avoid any further issues aroused from the IRS, I suggest verifying by reviewing the worksheet to confirm. You are correct that if he fits in the Qualifying Child category, it does not matter how much he makes as long as he can prove he does not provide more than half of his support.
The $4150 threshold is one of the requirements under Qualifying Relative. You will look at this one if he did not pass all the tests under Qualifying Child, for example, the support test. However, since he makes more than $4150, he would not qualify under Qualifying Relative to be claimed as a dependent.
I hope you find my information and explanation helpful!
[Edited 3/19/2019 12:43PM]
