Wise words by everyone.
I especially agree that retirement brings a sense of 'no longer being a contributor to the community or society' f
feeling which can lead to depression.
So I too agree that volunteer work could be an option.
Also, bear in mind, both of you, that whatever he does, he'll probably find the transition from work to leisure, quite hard. Most of us do and it's perfectly normal.
I don't know what his job was but is it possible to do it on an ad hoc basis? My retired uncle was a motor mechanic and continued repairing cars and bikes for friends as well as running 'beginner' classes in the garage a couple of nights a week for keen local teenagers.
So if he has skills or hobbies of any description that he can pass on, he may consider that. Do young people want golf instruction I wonder?
Choose the volunteer work carefully though, it has to be something in an area he would really enjoy.
After that initial period of getting used to not having the old routine, I am absolutely sure you will both begin to enjoy his retirement.