He is wrong.
If the student lives with you, you are the one entitled to claim him. Essentially, nothing else matters. Residence is the primary test.
The student is considered still residing with you, while he is away at school, even if living off campus. If he lived with you prior to going to school, he is still considered as living with you.
A child of a taxpayer can still be a “Qualifying Child” (QC) dependent, regardless of his/her income, if:
1. He is under age 19, or under 24
if a full time student for at least 5 months of the year, or is totally &
permanently disabled
2. He did not provide more than 1/2 his own support. Scholarships & GI Bill are
considered third party support and not as support provided by the student**.
3. He lived with the parent
(including temporary absences such as away at school) for more than half
the year
The residence issue alone concludes that you are the one who can clam him.
**Note that the support requirement is NOT how much support either parent provided. Even if your Ex could count GI Bill as support provided by him (and he can't) it would not matter.
