60 day visa for 15 year old to visit his brother?

Hi, a single entry L is enough for him, it you want him to stay longer, just apply for L extension from Beijing PSB before it expires. He can travel alone to Beijing if he is in possession of all travel documents (passport, visa, health certificate, etc.) required by applicable laws.
But different airlines may have different policies, you'd better contact the airline company for more details. As I know, permit by parents or appointed guardians is required to board the plane.