I think you can definitely tell him that he has nice eyes. I think anyone of the opposite sex would enjoy compliments. As long as you are genuine about the compliment :) Comments on the way of how you like his style, like his shirt or even the way he does his hair would be nice flirty lines to drop :)
make him feel good about himself. everyone like a nice person, and haven't you ever noticed started to like a guy just because he called you pretty or something? well its the same for guys. compliment then (not too much though, cause then you'd be a kiss ass). make him notice how nice you are and it should take off from there :) make sure you don't go into friend zone though. compliment him in the form of a flirt or such.