My boyfriend used to use excessive tongue and as much as I love him I was instantly turned off when he kissed me. But, recently he asked me what turns me on and what turns me off. I told him everything that turns me on and I also mentioned how I like to be kissed, and he asked "Am I a good kisser?" And, I just told him that I thought he could use less tongue and focus more on the lips. He didn't take offense and the next time I saw him he improved so much!
If you're gentle about it, a guy won't take offense, they mostly just want to know what you want and what you like.
If you don't have an opprotunity to talk about it with him without seeming harsh, then try this. The next time you're watching a romantic movie and there's a kiss scene, say something like "Oh, I love the way he kisses her so gentle" or something along those lines. Guys will usually pick up on that stuff, especially teenage boys because they are desperate for direction.
Hope I helped.