It's likely going to come down to the support test.  Make them do a support worksheet to show total money spend on his support and where it came from.  In the end math will tell you whether or not he provided more than half.

A quick Googling hit the worksheet from a VITA site:

https://apps.irs.gov/app/vita/content/globalmedia/teacher/worksheet_for_determining_support_4012.pdf

Rick