He may keep alternating his viewing of your Instagram stories to every other post because he doesn’t want to be too obvious about the fact that he may be intrigued by you.
In essence, he may not want to come off as too eager to know what you are doing in your life. He may be worried that this could make him appear as though he doesn’t have a life of his own and thereby desperate.
It was easier for him to look at your Instagram stories before the college semester started because he knew that he wouldn’t bump into you during the course of his day.
However, now that the semester has started, he knows that he will probably see you a lot more around campus and may be worried that you would think him desperate if he continues to look at your Instagram stories with the frequency that he once did.
This perception could make him feel very self-conscious. To an extent, he may already be feeling some of that emotion right now.
This may be why he would only look at you from the sides of his eyes while keeping his head straight when you said hi to him in class.
He may already be questioning whether seeing every other Instagram post of yours is even still too much.