Would you keep talking to this guy?

So this is a long story, but I need input. I met this guy at a program for my college over the summer. We texted all summer and into when school started. I live in the same dorm as him, but theres like 25 floors in my building, so I pretty much never run into him. Anyway, he had a girlfriend, and I was not interested in him romantically. He kept acting like he liked me, and I kind of pretended like I didn't see it. In like October he said he wanted to get dinner so I said okay, but he never texted me abou it. Like two weeks after that I saw him at an event for my school and he came over to say hi, while this other guy who I knew also came over. He walked away, and never texted me again. I occasionaly saw him walking around and he always seemed to avoid me, so I ignored him. Anyway, this semester, I've seen him alot more and it's weird. I really want to be friends with him again, I sort of miss talking to him. Tonight I past him, and he actually said "hey." So I said it back. But now I wish I said more. Should I text him? I want to be friends again. But it's so weird. Like I don't know why he was ignoring me in the first place, or why he said hey tonight. What could I say in the text? Pleeeease help me decide what to do :)