"Did [your son] file this return just to get a refund of withheld tax?"
What that means (usually) is: he did not have enough income to legally be required to file a tax return, but he filed anyway just to get the employer withholding refunded to him.
You need to coordinate with him. You don't decide on your own whether you can claim him. It's complicated but the bottom line (usually) is: you can only claim him as a dependent if his spouse agrees to file as Married Filing Separately (MFS) or he refuses to file as Married filing jointly (MFJ) with her. The one exception is they can file MFJ if they only file to get a refund, and can still be their parent's dependent.
For a full discussion see a similar case at:
https://ttlc.intuit.com/questions/2938663-can-i-claim-my-daughter-who-got-married-on-december-19th-2...
