I'm so sorry to hear about your son's seizures. I would like to invite some other Members of the Epilepsy discussion group to comment on this like @jakedduck1 and @lsittll.
Have you sought a second opinion for him? If not, Mayo Clinic has an excellent department for epilepsy/seizures. If you would like to get an appointment for him, here is the number, http://mayocl.in/1mtmR63.
I suppose the medical team at Mass General has tried different meds?