How to tell him I'm not interested

Note: I'm new to these forums. If this thread is in the wrong place, moderators feel free to move it.

---TL;DR version---
There's a guy who knows I'm gay. I get the feeling that he's either afraid I like him, or else he actually wants me to like him, but either way how do I tell him that I'm not interested? I don't want to flat-out ask if he thinks I like him - that would be awkward if he didn't think that at all! So how do I show my disinterest without verbalising it?

---Proper Version---
I'm 22, gay and a Mormon convert. I don't act on being gay. I'm out of the closet but I choose not to act on it. Does it get tough? Yeah, of course, but right now I'm happy with that choice.
Sam (not real name) is 19. He's part of my YSA group (a church group for 18 to 30 year olds). It's a pretty small group but we meet up several times a week.
Lately, he's been acting a bit unusually. 1. He looks at me when he thinks I don't notice 2. On the bus or when we're sitting across the table from one another, he "accidentally" brushes his foot against my foot before I move my feet away quickly. 3. Today we had sacrament meeting (mass). He was sitting next to me. When he got up to play the piano for the next part of the service, he put his hand on my lap as he was standing up. It was only for a split second but still, I don't know why he did that.
So I have two theories: 1. Is he afraid I like him and is doing this stuff to try and see how I'll react so that that way he'll be able to figure out if i do or not? 2. Does he want me to like him and is doing this stuff to get my attention?
Or is he just oblivious?
In terms of his own sexuality, he had a girlfriend for a few months a year ago but he doesn't anymore. This was the only girl he's kissed. People say he's straight but he plays the piano, wears bow ties, loves musicals and sits with his legs crossed so - although that's going on stereotypes - the jury's out on whether he really is straight or not.
But I have no interest in him. He's kind of cute but he's only 19 (three years younger than me) and I'm not looking for a relationship with anyone. How do I tell him that I'm disinterested without flat-out asking if he thinks I like him? Is there any way to show my disinterest without verbalising it? Or, if I was to talk to him about it, what would I say without offending him and ruining our friendship? He's one of my best friends in the group!
Please don't turn this into a "religion sucks" discussion. We're all entitled to live the way we want to!