What am i supposed to do?

Just tell him. If you keep it inside for too long he is going to hurt you even more and you don't want that. you already know how it feels to have your heart broken by him once so don't let him do it again! Ease him into a conversation just like you would normally maybe at a coffee shop or something and try, somehow get onto the conversation of relationships. Don't use the phrases 'the thing is...' or 'i have something to tell you' because he will freak out and make a bigger deal than it is. Just say to him '*Luke (or whatever his name is) i really like you and not just as a best friend but more than that. It hurts me when i see you and my friend flirting and i just think you have a right to know. I don't want to ruin our friendship because its so special so i haven't told you until now' or something like that! If he murmurs something and walks out don't panic, he just needs to get his head around it and he will soon call you up to tell you what he thinks. If he tells you straight out then again don't panic, if its good news then great! but if its bad news then don't worry just say 'ok thats fine, i just wanted you to know' then at least its out in the open rather than worrying you inside. The worst he can say is 'i don't' You will regret it more by not saying anything than telling him. Hope this helped and good luck :)