Especially while at university.... there's a lot of opportunity for him to have friends elsewhere. I had a bunch of courses where I never spoke to a soul - especially in lectures. Even in smaller courses, there were always people who didn't have any desire to really have conversations with others. It's fairly normal and common. Some people just want to go in, focus on the class, and get out.

There's even a chance that maybe he thinks you are trying to hit on him, and he doesn't want to encourage that.

I'd probably just continue saying hello when I arrived and then not bothering about it. I'm sure you've got a lot more to worry about than him!