Is my new client being manipulated by spirit meddlers to focus on dark thoughts and imagery, or is his mind simply distorted at this point to create such content on its own?

Both things are occurring. He has had attachments for an extended period. This has brought about changes within his mind, shaping him to be more vulnerable to dark impressions, to accept and believe in dark themes and potentials. This sets him up to be a mimic, in a sense, just as schoolchildren mimic their teachers in the body of knowledge imbued to them by their classroom instruction, backed up and enforced by tests of their knowledge within memory to ensure they are conforming to the expected inculcation of beliefs about the subject matter. In addition, there is an interplay with prior karmic history and that can be used as a theme and even weaponized by dark spirit attachments to intimidate and bully the deep subconscious mind by showing them their own past travails in other lifetimes and say: "You see, this is what's coming. You are in big trouble and you'd better start getting tough, otherwise you will get taken out," and so on. They will exploit any thoughts of their host that have the potential to be exaggerated, twisted in some way, to serve a dark agenda or add to ongoing fears and anxiety by raising the specter of dark doings that endanger the host. So, the Lightworker Healing Protocol is the very best way to begin to restore things. He needs this clearing and healing as well, to begin to undo the alterations and to help support him with protection, especially in the event of any backsliding with drug use.