I've had the odd grinding sound occur as well, I basically just wait it out til he's gotten it worked out and swallowed, and carry on
They've given him antibiotics for mild gingivitis. I will definitely try your food ideas.Has the vet said anything about his teeth?
In your first post you said he weighed 13 pounds, now your saying he weighs 9?
It is extremely concerning that he's lost that much weight.
Can you find a different vet?
I hope that you'll figure out a way to feed him a lot more calories. Try unseasoned boiled chicken with a little unseasoned white rice, many vets suggest to do this and since you're hand feeding him it doesn't matter whether he likes it or not, so get some kitten food and feed him that for the extra calories.