Maybe hes just a weirdo! Make a fake account, and follow him. Look into it, and just ask him hows he doing, and let him know you are there for him.
I have a friend thata into horror, and all that gory stuff. He's okay, and wouldn't hurt anyone, but still be be cautious. Maybe actually talk to someone. You need to find out if its just a weird thing hes into, or if its something more serious. Play stupid for now, and work on getting your facts. Make sure hes okay, and doesn't need serious help. He might be just a little weird or it might bw serious