It doesn't matter. If you tell him you still have a chance. If you don't, you will have no chance. To quote Yoda(yes, Yoda from Star Wars), Try not, do or do not, there is no try.
Tell Himm..
Iv Been Throughh It Beforee
Andd I Never Said Anythingg Becausee I
Thoughtt I Didnt Havee A Chancee..
Turnes Out He Really Likedd Me But By The
Timee I Found Out He Had A Girlfriendd..
So Youll Neverrr Know Unless You Trii
XD
If you know absolutely for certain that you'd be shot down, I don't think it's worth telling him. In my experience, The rejection hurts more then keeping it inside!