If it cannot be documented that one parent kept the child for more nights than the other one, the IRS would allow the parent with the higher AGI to claim the dependency. The amount of money spent on the child is not relevant. If the other parent files first your son would have to mail his return. The IRS would sort it out but it will be a mess requiring many months. The one who shouldn’t have claimed the child would have to give back any ill gotten refund and pay interest. Hopefully if this is explained to the other parent they will change their mind.
