It’s a deep question; can you get someone to change or “improve” how they do something sexual? There’s a self-help industry that’s predicated on the idea, but that doesn’t make it so. Let me give you two different answers to this question, the first is direct, the second is a little more circuitous.
Practically speaking you’ve got a few choices if you want to do a bad-kissing intervention with your boyfriend. You could say something directly. This might be awkward, hurt his feelings, and it may or may not help. If you’re going to take this route, think about how you would want to be told something like this. Also, think about how you and your boyfriend might be different in how you want to be told. I’m not necessarily recommending this approach, but in case no one has told you yet, you’re allowed to be direct, even about this.
You could also try to teach him by doing. Next time you’re making out, take control. Literally get on top and start showing him exactly how you want to be kissed. You don’t have to say anything, he may get it, and he may like it. Teaching someone how you want to be kissed can be, maybe should be, fun. Sex lessons can be erotic instead of didactic (except for those people who eroticize didacticism, when it's both).
You can also take the passive-aggressive approach of forwarding him a how to kiss article, but unless you both eroticize passive aggressiveness I wouldn’t go there. Also, it seems a bit cold.
But I think there’s more to be learned from this question. Think of this situation as a point of friction in your sexual relationship. There’s something you want that you’re not getting, which often means there’s an ideal experience you’re either imagining/fantasizing about, or recalling, and now comparing to your current experience. This point of friction is worth paying attention to and probing a little further.
My first question to you would be, is the kissing the only thing you don’t like? Would you describe you and your boyfriend as sexually compatible other than kissing? Depending on your answer to that question I’d have others.
This also feels like a chance to learn something about yourself. Can you describe what exactly it is that you don’t like about the way they kiss? Is it the technique? Is it the energy or intent, the timing or the pacing? A funny thing about sex is that because we’re not encouraged to talk about it much when we’re younger, we don’t always dig deep for ourselves, so something may turn us on or off, without us knowing why. We don’t always need to know why something turns us on, but if you can allow yourself to think about, and fantasize about the question, other desires may bubble up.
I’m not suggesting you need to interrogate your desires in a way that feels like you're forcing yourself to think about it. Instead I’m talking about being able to mull it over, letting your mind wander where it will but paying attention to where it wanders. If you can do that, even with something as seemingly basic as kissing, you’d be surprised what rises to the surface.