Bob was HIM
I'd venture that not only was Bob not real, but Bob was HIM as a younger man. Not much is mentioned of Qohen's past/childhood..
Some clues:
1. When Qohen first meets Bob, we learn Bob can't be bothered to learn names because it's a waste of human memory. This is a clue for how Qohen became lost as a young man.
2. When Qohen is in the park with Bob, Bob asks Qohen if he ever had a girlfriend. Qohen replies that he did... when he was about the exact age of Bob. Hmmm.
3. Bob's mysterious sickness that seems to get worse at the same time Qohen is getting "better." Because his need for Bob as a mental construct is becoming less and less.