Is it rude to do this? What should I do next time? Kinda long?

There is a rather handsome guy in one of my college settings...he is known for not talking much. In fact recently another girl wrote a note to him(I guess not all of us are very mature) asking him why he never talks...I wasn't there to hear/see his reply. But for whatever reason he is always flashing me this undeniably flirty smile. And once he blatantly did a double check to smile at me when I was walking behind him. Silly old me, I took this to believe he was interested in me, especially when someone else brought up a hobby of mine and he instantly jumped into the conversation and talked to me for a good 5-10 minutes. This is how the conversation ended---he asked if what I do is a partner thing and I informed him about another hobby that is similar to mine that falls under that category and I mentioned that I had invited someone else to come with me but they couldn't go. This is when he got kind of quiet and just nodded his head and looked down while smiling and said that he would think about trying the stuff I mentioned. Why do I say I am silly for believing all this meant he was interested in me? I found out that he has a girlfriend...who goes to the same college but is younger than both of us...but she seems very outgoing and apparently...she likes to do the hobby he was asking about. So I felt really naive for believing he was interested in me so when I saw him again for the first time since I found out he actually ended up sitting right next to me and he greeted me with a warm "hey" and I said "hey" back and that's it. We proceeded to go 3 whole hours not saying another word to each other. He definitely seemed distressed by something because he kept putting his hands on his head and leaving the room...but maybe he was just tired. I realize now that it may have been rude to not ask him how he was doing or anything, but he never asked me either. So I am wondering if what I did was rude and if I should try and remedy it next time. By the way that same day(that we didn't talk), when I first saw him, I was in a room adjacent to the one that I'm usually in and he walked by it twice, the second time he looked in directly at me and smiled flirtatiously. One of the main issues here is that I don't want to do anything that might make him think I like him because of the fact that he has a girlfriend and that's disrespectful. But at the same time I don't know how to talk to him without doing that because when I'm talking to him I can't help but smile. So my third question is, how can I be friendly without being too friendly? Thanks if you at least read all of this.