He can use TEXT messaging on a cell phone to another cell phone -- Don't know if 911 will accept it...

It is also possible to send text messages to internet addresses, so if 911 has always-on email, that would work.

He can make up his Help messages ahead of time and store them in the Saved Messages area of the phone, ready to launch.

I know 911 voice calls can be made from unsubscribed cell phone, but I don't know about text. ADDED -- Just checked with Kitsap county 911 here in Washington and 911 Center can't receive text or timely email, but that may not hold true in other jurisdictions.

Next alternative is one of those keyboard sets for the hearing impaired, where one can type msg to operator and she calls number and speaks msg.

There are a number of services that market Call Buttons that go to center to call 911 or whoever's on list -- They might cost money, however.

Another way, seen in senior trailer parks, is a daily sign in window stating "I'm OK" for a while -- If neighbors don't see sign at the right times, they do a welfare-check.