I have sent a text to my daughter to see if she is still awake (she is a vet nurse), but no reply yet.
Two thoughts ran through my mind when I read your post. One was that the cat may have eaten something like a plant. I had a Persian that ate a house plant and alas my vet couldn't save him.

I am not trying to alarm you, but if the cat has eaten something, the earlier the treatment, the better the outcome.
The other is if there is something wrong with his mouth. A year ago one of my cats was dribbling/drooling and acting quite strange. Then his tongue started to get locked and he couldn't eat. It would correct itself and by the time vets looked, there was nothing to see. Then one afternoon it did it again and I took pictures to take with me to the vet. Luckily the tongue was still locked in place and it was discovered his tongue was caught on his back tooth like a hook. He took the tooth out and touch wood, not a problem since.
It is probably neither of these things, but worth looking at the inside of his mouth and checking any plants for signs of chewing.
If my daughter comes back to me, I will let you know what she says - though I can guess she will tell you to see a vet a.s.a.p.