Q. So how does the IRS determine who gets it if its not addressed in the divorce?
A. As both answers have said; the IRS goes by who he lives with. From your description, that is you. While he is away at school that counts as time living with you, because school is a temporary absence. So, he has lived with you for more than half the year and is your qualifying child dependent.
There are two types of dependents, "Qualifying Children"(QC) and standard ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age limit, a relationship test and a residence test.
