Only in His Mind Series by Rebekah Blackmore

Erik thought that he had it easy once he left the Attley Mental Institution. He had Gideon by his side, he knew who he was, and most importantly, he had found a medication that made the hallucinations go away. When Gideon gets into a car accident, however, all that changes, and Erik is left to relearn just who exactly he is. Things get even stranger when Erik meets Myron Garvey, a man struggling with Alzheimer's, and the hallucinations become all the more real.