how old is he? there are two possibilities that come to mind if he is adult/old, these are bad teeth or an ear infection. if he is young then maybe teething or a non shedding baby tooth.
Other possibilities are a bit of something stuck in a gum, or stuck in his mouth.
A lot of cats dribble when they are being stroked. It can be a sign that they were taken from their mother too soon (same as sucking jumpers, etc.). It is quite common.
My cats dribble when they are having a wonderful time - it can be pretty unnerving if you are lying relaxed on the sofa stroking the cat, and it drools on you - lovely (not!).
We had a dear little cat who had no teeth and dribbled all the time. The trouble is she also liked to groom our hair whilst dribbling. She also did it to visitors. She would sit on their shoulders and do it. How to get rid of visitors!!
my boy cat dribbles when he is on my shoulder, he is purring at the same time so i think it's cos he is totally content. a bit like homer simpson when he sees doughnuts!!
Do get it checked out if he has only just started doing this. Yes...a lot of cats will dribble out of contentment.....but as others say-it could be a sign of tooth troubles
Also...and I don't want to scare you, but it could indicate worse things. I had a 5 year old cat that started dribbling,the vet thought it was her teeth, and did x-rays to be certain. Unfortunately she had inoperable oral cancer.