Sorry to hear of your son's loss Joann, and also to Silent Meow on your loss too.
When I bought my house nearly eleven years ago, it came with two (well three really) cats and they really didn't like each other because they were competing for food and attention, and one was neutered (Ginnee), the other not (Greizie). They both became inside cats four years later with a definite picking order. (They were both neutered by this point.) I had to put Greizie down last November and since then, Ginnee has not been happier and has been a much more loving (to me) cat and has noticeably settled down further. Like something clicked. None of the other cats that I have had to put down have made this impact on Ginnee (at one point there were eight cats in the house) and his attitude to the others that remain has also become more involved, with a whole lot of sniffing and kissing going on.
So I would say your son is right and Kibble isn't missing Chewie at all. And while it is very true that cats will purr for lots of reasons, incl being frightened and sad, and near death, in this instance I would say it is because there is no competition any more.
I would also discourage your son from getting a replacement pet. Let Kibble enjoy his cat-free home for a while at least. And if they really want another pet, get a female as Kibble might not see a female as the same sort of threat.
Very sad though...