If the company decides that they cannot keep the job open, they would dismiss him on grounds of capability. This is one of the valid (or 'fair' in a legal sense) reasons for dismissal. Redundancy is another one (of about six to total) fair reasons for dismissal. The amount of work required to be done hasn't diminshed so redundancy does not apply.
With a fair reason for dismissal, legally, what the company has to do is pay him for his notice period. Whether they do any more than that is up to them, the sort of company they are and (probably) how long he has been an employer - in other words, to what extent they want to be doing a decent thing. Depending on his age and his terms and conditions of employment, he may be able to take early retirement, or there may be a medical-discharge pension associated with his employment.