You tell him flat out to stop and that you won't take harassment lightly. I don't know how old you are - but if you are a young person in school and this boy is bothering you and will not stop then you need to bring it to the attention of administrators.
If he continues to harrass you to the point that you fear for your safety or he starts stalking you etc. then you need to report it to law enforcement.
Be very direct. Some guys have the idea in their head that if they keep persisting, they'll wear you down. They mean no harm they are just annoying. In that instance, you may just have to be willing to let him down hard if he won't take a hint. I know we tend to resist hurting feelings - but that could be sending mixed signals. Tell him in no uncertain terms, "I'm sorry, but I am not interested in you and you need to stop pursuing me".
Never change your dress or how you act - that's ridiculous. You are entitled to be who you are and you should not be expected to change yourself to "turn him off". You are a human being and NO means NO. If he cannot accept that and continues even after you have laid it all out for him, then seek help from outside sources.
Harassment is harassment and even if it isn't intended that doesn't change the fact that you, as a human being, have the right to say no and mean it.