My four year old is afraid to be alone in any room of our house-even if I am in the next room. What can I do?

I have to go to the bathroom with him. He has to go to the bathroom with me. I can't be around the corner in another room. I have to be in the same room with him at all times or he has a meltdown. I tried being tough for awhile, I tried catering to it for awhile, but nothing is working. I ask him what he is afraid of, and he just says being alone. He is too smart for a lot of the techniques out there like giving him a spray bottle and telling him he can spray away the scary things. I don't know what to do! I know kids go through phases, but this seems really extreme!