What in the world are you talking about?
People in their late teens are expected to feel sexual desires and want to engage in intimacy and sexual behavior. That doesn't mean they're "playing you". I'm befuddled how you mention masturbation, an almost daily, healthy, even beneficial act that most people do, as something unusual or negative.
If he's loyal to you and if he respects you, your decisions and your wishes, and acts according to that, what exactly is the problem here? If he was only with you for sexual reasons, why would he stay with you once you made it clear that you don't want it?
My impression is that you're quite immature and unable to handle other people as much as you want to be tolerated and understood yourself. Perhaps you need to grow up a bit before you can be in a proper relationship.