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, student status, a relationship test and residence test.
The support test is different for each type. The support test, for a QC, is only that the child didn't provide more than half his own support. The support test for a Qualifying Relative is that the taxpayer provided more than half the relative's support.
Since your son will be over 19, at year end, he must have been a full time student for at least 5 months to be a QC. His income must be less than $4200, for him to be a standard dependent ("Qualifying Relative").
