this lot: (I think there is only one company anyway under different names:
www.telecarechoice.co.uk/
it consists of a base station that plugs into the phone and a personal alarm which can be worn as bracelet (probably the best), pendant or belt clip. The alarm is showerproof and roll-over-in-bed proof so can be worn all the time.
should he press it, the base station will beep and whistle for 30 seconds or so and then someone from the call centre will call for him by name: 'Mr X, is everything ok?' He can then say what he wants done. If he doesn't respond, they will phone him. If he still doesn't respond, they will call whoever is first on the call list and then the ambulance. Have a secure keysafe installed outside, you can tell the ambulance service the code.
they don't mind false alarms and in fact encourage a monthly test. When you get it (which can be very quick), test it with him so he knows what will happen. Usually a night on the floor encourages it to be worn...
it needs a landline and obviously electricity - register him as vulnerable with the power company and/or the supplier.
£12 a month or so less any VAT discounts, sometimes free, but worth every single penny.