You seem to wonder why he put his hand on your thigh...
If he's interested in being your boyfriend, at some point physical contact will occur, holding hands, kissing, etc. But he understands you're shy, maybe he also is. So...
Situation "friends" would be you two sitting at opposite sides of a table, having dinner and an interesting flirtacious conversation...
Situation "in a relationship" would be, for example, you sitting on his lap, he's holding your waist and cuddling, etc.
But... how to get from one to the other, from his perspective?
Asking "Would you be my girlfriend?" would sound silly and awkward, plus it invites an even more awkward verbal rejection like "Let's just stay friends although we both know we won't see each other ever again..."
Also, since he's not sure you like him, going straight for a kiss like in the movies would be a very bad idea...
So he patted your thigh. If you had replied "don't touch me!" or tensed or had displayed other non-verbal cues of being annoyed then he'd have known you're not attracted to him, no need to explain or make a painful rejection scene. It's a part of your body that is still a bit "private" but not really offensive to touch (ie, not like your butt), so it leaves you the possibility to reject him without getting angry.
Apparently you accepted his contact, so he did it again, this time longer and more pointedly, and you also accepted.
This is why he touched your thigh three times, the first time just a brush or a pat so you could both think it wasn't intentional if you were annoyed or offended, this provides an easy way out of an embarrassing situation for both. Then the next times he added a bit more pressure to make sure you got the message. It's all non-verbal communication.
At this point he's a lot more hopeful about his odds of success I guess...
I at first thought that he was interested in me on a more genuine level, but with the way things have been going, I wonder if at this point he just wants sex.
You could be overthinking a bit. Well, in fact, a lot. Guys aren't that complicated ;)
Simply put: if he only wanted casual sex, he'd have pursued a woman more experienced, less shy, less religious, etc. Also the long and interesting conversations would have been shorter and less interesting.
You're 18, you say you never had a boyfriend, he's 21 so he's probably had a few girlfriends already, so he knows that even if you two have a perfect love story, it'll still take a loooong time before the clothes come off.
So to answer your question, I'd say he's a lot more likely to be interested in a genuine relationship. Which will of course lead to sexytimes, but you get to choose if/when/how that occurs.
Now it's up to you to decide if you trust this guy enough or not.