14 year old is acting strangely?

So , just over 2 years ago, he was one of the sweetest, most caring person we knew. But suddenly, he started to act strangely. He used to be very religious, even though we ourselves weren't religious. He just stopped praying one day, claiming religious is stupid, and he himself was an idiot for following it. He started having an interest in studying and video games. He stopped hanging out with his friends, and either was studying or playing some games. It went on like this for a year and half, but for the last few months he's doing literally nothing. Sudden bursts of anger, followed by laughter. He sometimes spends hours staring at a wall mumbling something and counting on his fingers. His teachers complain about him, saying he has great potential, but just spends time doing literally nothing in class. He ironically has all f's, but got into the top 3 percentile in the PSAT. We want to take him to a psychiatrist, but he refuses, saying he's just thinking about something and not mentally ill. What should we do with him?Edit: Also, forgot to mention his "personal" notebook. Along with drawings of penises, there's a 2 page essay which is literally a rant about his existential crisis. It contains the human size scale to that of the planet, stars and the whole universe and how insignificant we truly are. via /r/Parenting https://ift.tt/2xdxh26