Not that simple, but Doro 603 can be used that way - open lid to switch on, press and hold button A to ring person, red telephone to switch off. Or is the problem that he cruises through buttons and changes settings? I don't think he'd do anything irretrievable on the Doro. And of course sticky labels help.
The Doro also has an alarm button on the back - just push, and it dials number (or several numbers in sequence. but for alarms, there are alarm buttons on a necklace or bracelet that connect via landline system to call centre, who establish whetehr there's a problem and then ring family - LAs often run these at low cost. And other alarm systems that connect via mobile.
There is probably a tracker available (1) - alternative is to use mobile phone, but encase it in a sealed shut cover so he can't meddle. there's certainly motion sensors which will alert you if he, for example, leaves the house. (1) - you can get them for your pets so you must be able to get them for people.
I usually find out about stuff like this by googling, eg alarm dementia.
Alternatively - Alzheimers website, Age UK website, local disability shops/centres which you can find by googling, Occupational Therapist. There's almost certainly going to be some sort of dementia support group near you so get in touch with them as well.