If you want to hear my advice, i actually have went through this, it is terrifying, some of it is just in there head, like constantly worrying that demons or people are reading their thoughts, but other things like seeing visions of shadows, demons, etc. In my opinion are demons, but the thing is you either have to be somewhat connected to evil, lets say through the wrong crowd, or by just looking at an evil person and that person from then on would see that person in their head, which would probably be influenced by a demon.
In other words, the fear they feel or the constant anxiety is sorta them but the visions and things they see are demon related and also psychological.
I thought it was shcizophrenia but i took loads up medication and would still see evil visions, some very very strong and scary and some also gave me pain. An example of a vision was way back when, when i was with the wrong crowd, the party crowd as you can imagine…Me and my roomate had people over, some were pagans, and one a satanist. When he left that night, note that i did bad things i regret, we were all going to sleep, i slept with the light on due to extreme terrors. I had many terrifying visions that night, some were when i closed my eyes, i saw a hand creep in from my door that had painted nails, another was a girl said in an evil voice this is going to hurt a lot, i felt a horrible pain all over my body. Others were a crazy man laughing and turning his head around, it terrified me.
You can believe me or not but i saw what i saw. I am guessing it was a punishment from god when i saw these scary things, I could understand why though… i was still in a bad spot in my life, i was converted but still around a lot of evil. Anyway i will pray for him but i also reccommend you ask him if he is in a bad crowd or around “bad” things. Or at least does things that may make him worthy of punishment.