My friend stares at my chest a lot! Does it mean anything?

Whenever I talk to my guy friend I notice him looking at my chest a lot. Sometimes its like five times per conversation. He usually looks for about 15 seconds. He is actually a really attentive listener and makes direct eye contact when we talk about 90% of the time. Is it just a male thing? Sometimes when I see him looking his eyes are open really wide and it's almost like they're bulging out of his head. He's in his mid 20's and he has a girlfriend. He never sees me catching him stare. Does it actually mean anything or is it just that I'm a girl and he's a guy. It doesn't bother me, I just want to know if it could mean that he likes me or is it just that he is a guy? Also when he stands up he will adjust his belt buckle. I never notice him do it around other people, but he does it whenever we're together. Someone told me that he could be trying to draw my attention to that area. Maybe it's just subconscious?