Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern for your cat although I'm glad that the excessive salivation doesn't appear to have affected his appetite.
There actually could be several different explanations for this behavior; there wouldn't be just one cause. In no particular order, they are:
1.Problems in the mouth such as an infected tooth, ulcers, abscesses, foreign body. Sometimes a thorough oral exam will reveal the problem although sometimes sedation may be necessary to adequately examine the mouth.
2. Ingestion of any noxious substance that he might have groomed off of his body (such as flea/tick products) or anything that he might have licked or ingested when he was outside.
Flushing his mouth out with cool water was exactly the right thing to do although there may still be a residual bad taste which is why he started to salivate again.
3. Nausea for any reason. Systemic diseases such as kidney disease or inflammatory bowel disease can cause cats to feel nauseous. He's a little on the young side for such a problem but I include it to be complete.
4Early upper respiratory infection or Calici virus.This virus can attack the mucous membranes in the mouth and create ulcers on the hard palate and tongue; they are very painful which is why cats with this problem will excessively salivate. However, I would expect you to be able to see these ulcers; I also would expect him to be sneezing or have a nasal or ocular discharge.
5.Spontaneous.In some cats, we can’t find an explanation and they will spontaneously stop drooling and we won’t have had any idea why they started in the first place.
If this behavior continues, you can give him over the counter Famotine (Pepcid AC) at a dose of ¼ of a 10 mg tablet twice a day which may be helpful. This drug may be difficult to find in the UK, though; in that case Omeprazole (Prilosec) is an acceptable alternative at a dose of 1/2 of a 10 mg tablet once a day.
I hope this helps and that this is just a transient event. Deb