Shared ownership.

Sounds like he needs to cut his cloth to suit his wallet I am afraid. What does he earn, what does he have saved, and what are flat prices like where you live? (Does he really need a house??)

I agree about posting elsewhere, but will add: all of us caution against getting in over your head with financial commitments. Whether the mortgage provide would allow it, would depend on how they assess his finances.
It sounds as if you, yourselves, have little capital, and would rely on your income to top up your son's payments. This is not completely daft, but you need to consider:
the alternatives (as suggested)
how secure are your jobs?
how secure is your health, and what would happen if you became unable to work, or died?
What changes in 3 years to make this more workable?
What is the 'fallback' position?

One of my family did something similar, in a situation where:
renting was difficult, commuting unsustainable (unusual job)
parents were in good health with secure jobs
3 years (same timetable!) would see a significant rise in income

It worked, but it was a gamble!

His entry plan requires support for deposit and running costs.

What does the exit plan look like?

Without one that is viable expect to be paying into this for a long time.