Description

 interested in. Some ways to figure this out would be: 

Observing his clothes. T-shirts with band names or pictures on them may be a dead giveaway to what he's interested in, so keep your eyes peeled. 

Studying his possessions. Don't become a freaky stalker, but try to get a glimpse of his bag, or something like that. Why he has a bag, I don't know but if he does just walk away. But what's on there may also give you a clue of what he's interested in. 

Observing his friends. It might help even more if you know them. They might have the same interests; you can look at what they do together-listen to music, talk about a certain band etc.