Primary Progressive MS, paralyzed entire body but head and neck...how long to live?

Otherwise healthy 50yr old man, diagnosed with PPMS, can not move any body but head, Supra pubic catheter, living at home, caregivers around the clock. Eats well, sometimes coughs but is on thickener for think liquids. I am wondering what is to be expected in the future based on any other statistics. He is able to use Facebook by means of head control of phone, and can eat as long as he is fed. Rarely coughs when eating. HAs trouble with bowels because of sedentary situations On Vicodan one daily He takes Miralax 3x per week, and uses a hoyer lift to hang over a pot.He is not bowel incontinent, as he can usually feel sensation of this

urgency I would like to know what the life expectancy is with others, as it would be nice to be able assist family in planning for other devices and help. Many thanks.

Hopefully as long as possible