Ok since you only talk to him through facebook and not face to face, I'd really say that no matter what you say it will be fine because it's not face to face. But ok I guess you just need to catch him next time on facebook next time and then tell him, I would say sth. like this, "(formal greetings) so, there's sth. that I want to tell you (he responds), well I hope this doesnt ruin our friendship because you're my best friend, but now that you're going off soon, you need to know that I like you very much and not as a friend" well that's very direct and you asked how to "kinda" let him know maybe bring up other boys you like and find a way to flirtatiously tell him that he's cute and would make the perfect bf... idk I'm just getting started with this, I moved half way across the world and completely find gorgeous the boy who sits next to me in most of my classes and I swear he gives me signs that at least he likes me as a friend... It 's hard haha, god bless and good luck!