The wallkie talkie idea is a good one. When I was young, I had a friend whose bedroom was a converted shed out in the yard. (He liked the privacy it afforded.). They had a radio intercom set up, base station in the house and the remote station in the bedroom. Something like that might work, and the simplicity of the large single button might be good for him. (Edit: Modern version here, claims pretty good range.)

Another idea: I recently got a remote control electrical plug thingy originally intended to activate outdoor Xmas lights. I use it to activate a "recording" sign downstairs from my upstairs studio. Plug a light bulb into that puppy and he can signal you if he needs you.