Well. A good question.
I confess I'm rather testing the water here and this is just my experience and opinion, but I'm not a fan of vaccinations.
When I was growing up with cats, and as a young adult, I understood there were two vaccinations that a cat 'ought' to have - one against Feline Infectious Enteritis and one against Cat 'Flu. Trouble was, FIE was only supposed to strike cats under one year, flu was to have so many varieties that it was impossible to expect the vaccine to protect against all of them (we lived in terror of calicivirus, chlamydia and the rest) and it was even said that apparently FIE and flu were supposed to be the same thing.
I saw many kittens with what I feared was FIE - and later realised was worms/teething - which can cause diarrohea and the awful vomiting and dehydration that can kill...sadly I lost three little rescues before I read a newspaper article about dehydration in babies and learned about dehydration, electrolytes and replacing body fluids (in those days it was saline/glucose; these days there are more sophisticated methods.) Vets just doled out antibiotics, which didn't work. Later I was in touch with a brilliant homeopathic vet who recommended teething granules; don't know if you can get them now but they do the trick with a teething kitten who's passing smelly sludge and doesn't know what to do with itself!
I've only ever seen one case of cat flu. This was after I took a cat to the vet after a mild road injury...in those days (1970s) waiting rooms weren't so spacious and cat carriers weren't insisted on...I was squashed up next to someone with a sneezing cat in an open box...yes, I know, thick of me not to have moved straight away from the poor creature! The cat and her mother came down with the lurgy.
In the 80s when I bred Siamese I had my queens vaccinated and kittens also, it was regarded as essential and responsible. Also no stud owner, rightly, would accept a queen without vaccination and blood tests for FeLV and later FIV.
When I stopped breeding I didn't bother with vaccinations; the cats were getting elderly and didn't roam. When I moved to my present house and lost my old cats and became invaded by needy ferals lol I was like you, with cheeky freeloaders dropping in for a snack and now I too have a couple of cats in their teens, who've never been vaccinated.
These days the garden is secured (Altracatz!) but all the cats once roamed on the streets. Despite no vaccination, they're all in good health (paws crossed!)
I believe there is a vaccination against FeLV too nowadays and is there a vaccination against FIV too?
But in my opinion, if your guy has got to 13 and built up his immunities, then he ought to be ok.
However, always worth consulting the vet - and also other people on here may have different opinions - I and my cats may just have been lucky; I don't claim to be an authority!
Oops, this has been a 'novel' - one way to deal with the dreaded insomnia!
There will be other opinions on this but whatever, hope Sonny has many good years and times ahead of him and Pepsi too!
