Seemingly uncomfortable doesn't mean he's using you. The fact that he seems to be opening up to you more, and sweet suggests that, while he may have a hard time trusting opening up his feelings to a women, that' he's learning to - and that's a good thing. Take it slow and realize that it takes time to develop intimacy in a relationship.
Guys at 21 are often emotionally immature. Agree with anniebrooke that he may be just not be great at communicating. Give it time if you like him to see what happens. Is he in college and you are in high school?
Maybe he is just feeling awkward about the whole thing. That doesn't translate into him being a user, just a typical not-quite-grown 21-year old.