You smile at him sweetly and say, 'Look Honey, I really like you. You are very attractive to me, but this kissing thing isn't working for me this way." Then you tell him his kisses are too wet or too hard or there's too much tongue, or whatever,for you to enjoy them. If that sounds too harsh, tell him that they are so much a certain way that you find it distracting. Gently. Then you ask, "Can I show you how it would be better for me?" Then show him and ask if that is OK, saying that you want both of you to enjoy it.
That's always been my approach anyway, and I have never had it fail. And I've had guys who kissed like wet mops, ones who wanted to stick their dripping tongues in my ear, those who thought they were Hoovers, and one guy who almost choked me because he was trying to lick my tonsils.
Good luck. If he's a decent and respectful guy, he'll be OK with what you need and try to adjust his methods.
Chances are he was nervous.Spend more time getting to know each other then when you get to the kiss you can smile and say relax we can go slow.
Tell him he needs to change the way he kisses and if he doesn't like it then tell him he can kiss your a s. No just kidding. Seriously, not sure how old the dude is but maybe he just needs some practice. :)
The best bet is to whisper little things during like "softer" "slower"
That usually doesn't come across as insulting. Some guys like to take direction so you could also just say "kiss me like this" and he should respond. Communication is everything. Some people don't know because they just have never been told by anyone. If you like him, it's worth the effort. When he does something you like tell him so he'll know next time.
You should tell him how you feel about his kissing a kind way, so he doesn't get the wrong message.