9 Answers

I think the first thing you must do is discover why he is so unhappy, then work from their. perhaps he got troubles on the home front,school who knows but in order for him to be happy you got to get to the root of the problem..

Make him laugh.......and then talk with him....

BUY HIM A NEW HARLEY DAVIDSON

You have received very good advice from daren1.  Adding to that, it would be a good idea to approach the subject by saying that you have noticed that he seems unhappy and depressed lately and are concerned about him.  Offer to be a good listener if he wants to "unload" and then be just that.  You don't need to give him advice or try to make anything better.  A good listener can make all the difference in the world.  
Please let us know what happens.  It is always a concern of mine when someone so young becomes unhappy and depressed.  
If you find his reasons alarming (drugs, abuse, suicidal), please find someone you can trust with the information (NOT a peer; an adult).   Call the local police department if nothing else and ask for a referral.  You should not have to give your name or any other info.  
I hope he's just stressed over a math final. 

You are too young to be going out with boys. Perhaps he realises it, love, and doesn't know how to tell you. Ask him what is wrong.

I wish you all the best here . May this go well for you .    Bill

At 14 years of age, you are way too young to start getting involved in drama and trying to "make someone happy".   As a child, that is not your job.  If you are concerned about him, as a friend, then express your concerns to an adult and let them handle it....one of his parents, one of your parents, a teacher, a school counsellor.  Do not think that you can "fix him"...you cannot.

By experience it may mean that he isn't that much into you anymore

a lot of sound advice given to this question