Kill Switch, by Chris Lynch

Danny has always been close to his grandfather, Da, and during the summer after his senior year, Danny wants to spend as much time as possible with Da before leaving for college. Da, however, is rapidly losing his memory, and keeps telling the family stories that don't seem to fit in with his life as an agricultural department systems analyst. As Da becomes more loose-tongued, and ex-colleagues of his start coming around, Danny realizes that Da's wild stories about assassinations and so on are true, and he takes Da and his stoner cousin Jarrod on the run, in an attempt to keep them all safe.