"While you travel with the boy, the man in black travels with your soul in his pocket."
The theme of The Dark Tower the Gunslinger is survival and the quest for knowledge.
Development of Theme #3
"The man in black is your gateway to the three"
"SATAN! Someone was screaming: THE ACCURSED! BRING HIM DOWN! THE INTERLOPER! another voice cried."(King 59) Within the scene, a preacher who was turned by the antagonist of the series, convinces the town that the Gunslinger was staying in to try to kill him by personifying him as the devil. The gunslinger then defends himself and kills all of the town. Doing s shows his theme of survival mentioned prior.
Conclusion of Theme
"Go slow past the Drawers, gunslinger. While you travel with the boy, the man in black travels with your soul in his pocket."(King 90) This quote is a "spirit" talking to the gunslinger, and the "spirit" had the voice of a person he knew in Tull, or the place where he killed everyone. The spirit is trying to confuse the gunslinger with a slight riddle. This helps to convey the theme of the gunslingers quest for knowledge by making him question it and go after him.
"The boy is your gateway to the man in black. The man in black is your gate to the three. The three are your way to the Dark Tower."(King 130) This quote shows that the gunslinger brought along the boy for a reason on his quest for knowledge, as a sacrifice most likely. To get to the tower, which is a part of the quest for knowledge theme, he also needs to find the three. The three will help him get to the tower and then his theme of a quest for knowledge will be complete.
I believe that the solution of the book will be that the gunslinger will find the three and be able to go to the "Dark Tower," Then there will be another book, in which the gunslinger follows through his actions within the Dark Tower. This is my prediction, which is most likely right. And a bold prediction is that Jake(the boy) dies.