Yep.  It is not refundable so it just offsets tax owed on Line 19 of the 1040.

I agree with your answer, but what concerns me is that the son is age 19 and not a full time student. He has no income and lives with his parents. He does qualify as a dependent as a qualifying relative. Since he is a qualifying relative, is his age not a problem?

Here is info from IRS  https://www.irs.gov/newsroom/what-taxpayers-need-to-know-about-claiming-the-credit-for-other-depende...

also Pub972  will help answer your questions

Yes age is a problem. Additional CTC can't be claimed of dependent is over 17.

However it sounds like you may benefit from the Credit for Other Dependents
https://www.irs.gov/newsroom/you-may-benefit-from-the-credit-for-other-dependents

Thank you.  Pub 972 was very helpful and confirmed as I was almost sure that in this situation, I would be able to take the credit.  

Happy that I was helpful

Which credit are you planning to take?
Don't think you can take the additional $500 CTC

From Pub 972

The ODC cannot be used to figure the ACTC. Only your CTC can be used to figure the ACTC. If you are claiming the ODC but not the CTC, you cannot claim the ACTC.