How is residency established if parents are divorced

start here
http://www.umass.edu/dean_students/campus-policies/residency

MA Rules and Regulations for Governing Residency

While you and your ex may have joint custody, for college related things (filing the FAFSA, CSS profile, determining in-state residency, etc) the custodial parent will be the parent who the child lives with.

Your son, may still end up getting in-state tuition through your residence in MA. Your best bet would be to contact UMass Amherst to find out their policy as it pertains to your situation.