Don't make a big deal of it or it will turn into a game and he will learn that if he does it he will get lots of attention. Just ignore it and gently move his hands away. He'll soon stop it. Or pre-empt it by holding both his hands when you kiss him.
agree with angeldog - demos are the way little ones learn. Stroke his face gently, get him to touch your face gently, "oh, thats so gentle, thank you, thats lovely" with big smiles etc.
My DD is almost 2 and she still scratches faces - sometimes when over excited, and sometimes to get attention (in which case she gets a 20 second time out, as well as demonstration of 'gentle', and is made to appologise).
She is an otherwise delightful child! I say this not to discourage you (although yes, it might take a while to teach him!), but to demonstrate that its not an uncommon thing for babies and toddlers to do. I'm sure your DS is lovely
It's horrible, DD went through this phase too. I'm only half joking, BTW
If kissing him provokes a violent reaction, then the easiest, least confrontational course of action is not to kiss him for a while- difficult, I know! It is only a phase though, and soon enough he will be planting smackers on you all the time.
Otherwise, I agree with modelling gentle touch for him and not reacting when he hits you.