Who gets to claim a child on their tax return?
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.