How old is he?
does he go outdoors?
is he eating?
You still need to keep an eye on your boy for inappetite,cold temp, etc
He may have picked up an infection from his buddy.
Cats do not have whims like people. Whenever they stop eating they are either ill or in pain. If he has no other symptoms like fever, diarrhea, constipation, infection, etc then there is something going on that is difficult to ascertain - but it exists nevertheless. Sometimes a cat can have an abscessed tooth or root that is in the gum.They can ingest a piece of plastic that in not perceivable in an xray. They can be mildly dehydrated or have a metabolic or organic imbalance that is too slight to read on a test. He may also feel nauseated.
If he stops eating again and your cat is not diabetic begin by giving him two droppers (cc size) of unflavored pedialyte every ten minutes for an hour. Next water down(plain water) canned cat food or baby food chicken or turkey with no onion or garlic and give only one dropper every ten minutes for another hour. Wait two hours and repeat both. When he feels hydrated he will lick up the gruel on his own.
Boil chicken or fish as the scent of food cooking awakens the appetite sensor in the brain.
Please get started ASAP and let me know how things are going. I hope this was just a temporary upset and Freyr will be fine.
You can check for dehydration by pullung up the skin at the scruff and see if it springs back or moves slowly. .
I would not worry about diabetes in a cat his age.
A cats appetite is the best barometor of health because sick cats wont eat so as long as he continues to eat, he is fine.
Drooling can be a sign of a bad tooth, nausea or licking a distasteful item.
Let me know if you still have questions.
I appologize if I did not fully answer your question. I was just leaving to go out on calls.
If you need anything I will always be here for you.
God bless you for taking this sweet kitty in!!