Our son is a resident of GA, but a grad student in NC and has earned income in NC since late May 2016 - Does he file as a resident of NC or of GA?

He remains a resident of GA and should file a GA resident tax return and include his NC income on that return.

Depending on how much he earned in NC and/or if NC taxes were withheld, he may also have to file a NC non-resident return. GA will give him a credit (towards the GA tax) for any tax he pays to NC on the same income.

Make sure to prepare the NC return first so the program will know how much NC tax he can credit towards his GA return.