I'm gay and in love with my straight best friend. He knows I'm gay and flirts a lot. Should I tell him that I'm in love?

The truth is better out than in, I think you should go ahead and tell him. If he's a true friend then he'll either let you down gently or just take you up on your offer.

How does he feel about his own sexuality? It is better to tell somebody but you have to think about his reaction in case you lose him. You don't want to do that.