I was worried when my father retired, especially as he'd been widowed just before then too. But it has been OK
Each day he reads the newspaper from cover to cover, does the crosswords etc. He then does something active - gardening, golf, DIY, weekly shopping, visits library. Then lunch and nap, followed by dog walk, phone calls, supper and TV.
He is also on a couple of committees, plays golf twice a week, has a regular slot with mates at the local pub etc. Sky sports has been a blessing in bad weather too.
Best way to ensure lack of boredom is to have an ongoing project - build a greenhouse, make raised beds for the garden, research a new car, restore a bit of furniture, that sort of thing.
They both got involved with a charity, first giving practical help then joining the committee.
And they signed up for a number of classes - talks at the zoo, nature walks, art history.
They were also regulars down at the local library.
I think keeping physically active is important, so if your dad already has a hobby that keeps him active he should try to keep up with it. If not, take something up - golf, gardening, hillwalking if "sport" isn't his kind of thing.
And keeping mentally active, plus speaking to people. Even just popping down to the shops to buy a paper instead of having it delivered.
Thanks all... I'd not heard of the U3A and looked that up for our local area and there's lots going on. I've sent him some links and I'm sure he'll find something to do. I think he's just feeling a bit and anxious that it's the end of an era, and that he's signing up to old age and getting closer to dying.
I think men, particularly of his sort of generation, were often brought up to define themselves by their job, so it can be a real "identity crisis" to go from being "a bus conductor" or whatever to being retired. IME women don't usually define themselves quite so clearly by their job, as they have usually spent their working lives feeling split between "a bus conductor" and "a wife/partner/ex-partner" and "a mother, daughter, sister" etc so retiring just means giving up one of the things they have been juggling and not the thing that most defines them.
He might well find that taking on a responsibility for something gives him more sense of purpose than any amount of hobbies and "keeping busy". For my dad that was putting his IT skills to good use setting things up for a charity, where he really felt he was being useful to them rather than just filling in his time.