Oof, OP I don't want to break your heart but even tho he is YOUR best friend, you might not be HIS... anymore. Your replies are like "he doesn't have many friends," and "I know he wants to hang out with me" etc. But in reality, when people want to see someone, an actual effort is made without asking for it. Because they genuinely want to see you without you telling them over and over. He... doesn't.
You're at the age when people begin to drift apart due to normal life circumstances. Also, you can't expect one friend to fill up your social loneliness. You gotta get out there and make more friends! because this is when you begin to make connections and memories that will last for a life time.
Accept this friend of yours might grow even further apart from you and holding on to it will only hurt more. Good luck!