Even if you did not use the 529 distribution for education and your son had to report the 1099-Q as income, he would still be your 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 (under 24 if a full time student), a relationship test and residence test. Only a QC qualifies a taxpayer for the Earned Income Credit.
Further more, although the IRS has not issued formal guidelines, it is generally accepted that support from a 529 plan is considered as support provided by the parent, since the parent is the owner of the plan.
