He apparently qualifies for the credit. Click on this link for the NY website that discusses this credit (or itemized deduction, if that is a more favorable tax deduction): College tuition credit or itemized deduction
You are correct that he must be a full-year resident. But if his absence from New York was for school, this is considered a temporary absence which does not constitute him a part-year resident of another state. If this is the only concern, you may both file a joint return and claim the credit, because nothing on the site says that he must be attending a NY school.
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