Instead of simply sitting idle and assuming things that may have caused him to stop chasing you, it is best to get to the root of the problem by confronting him about it. Ask him directly if something is wrong or if there is something that he is mad at you for. There is a good chance of him being mad at you because of some sort of a misunderstanding. Letting the misunderstanding grow will only turn him bitterer towards you and keep on making you feel that you are at fault or that he does not interested in you anymore.
Establish and maintain a “No Contact” rule. This is going to prove extreme tough in the beginning, since you will be craving to know why he is avoiding you and not showing the same sort of interest that he did before. However, you cannot give in to your curiosity, as it may make you come off as desperate. Do not send him a text, an e-mail, a Facebook message, or send him a message through a common friend. Do not let him think that you are missing him badly. Remember, you want him to be chasing you and not the other way round. He will start taking you for granted if you show even a hint of desperation.
Distract yourself from his thoughts by going out with your friends and having a good time. There is absolutely no need to punish yourself by stop enjoying life. Watching a movie or developing some hobby is an effective way to keep yourself distracted.
Remember to keep your self-respect. Do not become desperate or weak. If he does not contact you for a couple of weeks, let it be. If he does not care enough to contact you himself, then maybe it is time for you to move on and find someone who actually cares and consequently deserves your love.