What you are missing, as asked, is that whether a flat or a house, they have to pay a premium for that extra 50 years or 90 years.
When he purchased in the 70's the problem with leasehold houses was already understood and the Leasehold Reform Act 1967 in place. The advice would have been given that it was a wasting asset and that the lease should and could be extended. it was likely cheap for a reason..............
At the end of the lease there is no new lease. He is entitled to stay as an assured tenant at a Market rent not a ground rent, as if he was renting in the private sector albiet with greater security as as an assured tenant, not assured (shorthold).
Why would a property developer buy it with a 40 year lease?! the owner might want to pay him to get it back early if there were development rights that he could exploit, which would increase the vlaue, if you can persuade him to sell. That at least gives a cash sum to acquire or rent something long term say shared ownership.
Harsh it may seem but when he bought it it was what it was a short 56 years lease that would cost a lot to extend or there would be nothing for his family to inherit. Nowt to do with fairness it is wata it is and you make your choice....
Based on the information posted, I offer my thoughts.Any action you then take is your liability. While commending individual effort, there is no substitute for a thorough review of documents and facts by paid for professional advisers