He's gone downhill. Help?

Hey guys.
As most of you might remember, I have this crush on a guy in school but it's slowly passing now. But I still dunno about his current situation.

Ever since Christmas, he has been acting differently. He went out with some guys from class and apparantly one of them did a personality switch and I'm suspecting he went out most on my crush. However, this is just an assumption due to the fact that when my friend and I asked him and his friend what they thought of the other guy he was the one who was coming out with angry remarks and saying that he hated him. Ever since that night, he's been really cold and snide. He snaps at the guy which is a bit understandable if he did actually go for him but we're still unsure. He's also began to be a bit snappy with the lecturers in class and some people, saying horrible things about them when they're gone and it's just us in the room. He's also been going out drinking alot for the past few weeks now. He was out on the weekend and then was going out on the Wednesday after college and then the Friday after which would result in a full weekend binge most likely. I also heard that, last week, he was told off but just ignored the lecturers and continued what he was doing despite them telling him to stop and listen to them. When he first came into the class, he was a really sweet guy who was always laughing and smiling. Now, he's just gone completely downhill and is complaining and frowning alot. He mentioned the other day that his cat got put down and he told me the other week that his dad isn't working due to an accident which left him with a bad back which stops him from doing alot.

Should I just stay out of it despite being really concerned? Is it him rebelling [he's just turned 19]? If I should help him then how can I actually approach him?