Each school/system has its own method of determining state residency for tuition purposes, which may be entirely different than what constitutes residency for other purposes (taxes, voting, etc.). Once you determine what college your son is interested in, you’ll need to go to their website.
For instance, here’s a link with info on residency status for UMass Amherst http://www.umass.edu/dean_students/campus-policies/residency
Hope that helps.