From OP,

I know this because the evil beings will say he's going to do something and he will right then and there[do it].

If OP is truthful, this is an important clue, that tells me this is a legit demonic attack.

The demons will access to information, that would have been impossible for you to know about, but, which can later be verified to be true.

No mental illnesses, can explain this.

Once when we were going to the place of prayer, we were met by a slave girl who had a spirit by which she predicted the future. She earned a great deal of money for her owners by fortune-telling. - Acts 16:16