I transported three elderly cats (one brother and two sisters) to New Zealand – quite an adventure for them. It gave them all a new lease of life and, surprisingly, they settled into their new lives very quickly. There were all old and not sprightly but were doing okay and seemed contented. Dingbat, aka Fatty, who loved having a large garden, was nearly 18 when he started fitting one evening and, in spite of overnight care at the vet, didn't survive. Bodoni made it to 19 until she became so frail and muddled that it was kinder to put an end to her life than keep her hanging on. Frutiger managed to get to 20 before her life came to an end.
Cats have short lives compared with ours and any cat over 17 – quite a good long life – isn't going to last a lot longer, although there have been some spectacular exceptions. However, we cannot count on those.
I would say, keep an eye on the cat but if he is not distressed, let him be. If he becomes distressed, stops eating, stops drinking, or shows any other signs of difficulty functioning or pain, then contact the vet. I would then suggest that you allow yourself to be guided by your vet's advice and, if he thinks euthanasia is the right option, then be brave enough to choose it. It is a very painful decision to make but we pet owners and animal lovers have to be strong and more loving than we have been. Keeping a suffering animal alive is not a kindness. I think, maybe, you should say all this to your son.
I have had to say goodbye to many much loved cats over the years and I know how terribly hard that can be.