The one who provides the majority of the economic support for the child is generally the one who gets to claim the child for tax purposes: claiming a child on your taxes, after all, offsets the monetary/financial cost of the child, so it the person who provides the bulk of that support who should get the benefit. If the $650/month would be more than 50% of the money spent on the child, he should get to claim the child; if it's less than 50%, you--who would be providing/paying for the rest--should.