There are various possible causes of drooling in a male cat.
1. Dental disease - there may be other signs such as pawing at the mouth, slow eating etc
2. He may have licked something that has caused him to dribble - if he has been outside and tried to catch a frog or similar then this can cause excessive drooling. The same can be said for if he has groomed himself a lot and you have recently applied a flea treatment to him.
3. There may be a female cat around causing him to hypersalivate - less likely if he is neutered. Or other behavioural cause
4. Upper respiratory tract infection, foreign body or underlying disease causing a difficulty in swallowing - usually accompanied by other signs other than just salivating a lot.
If he continues to salivate a lot then a check up by your vet is recommended as they can closely inspect his mouth etc for any gum disease, lesions on the tongue and give him a thorough clinical examination
By Company Of Animals (Professional Advisor)