Where his official residency is. If he moved out of state to go to school and wanted to change his official residency then he would have to look at that states guidelines and officially request to change it, ie go to the DMV or get a residency card from that state. He will likely be a resident of "whence he came". As always check with your accountant. 
Because my son moved from the Peoples Republic of California, he changed his residency to where he spent the most time. (and had no state tax) He had to go down to the DMV with the appropriate paperwork proving he was a resident.
