Bi-Polar is the new one size fits all shrink diagnoses now.
If he finds a voice inside constantly accusing him and reminding of every humiliating and wrong thing he ever did in a way that it paralyzes him or depresses him, it could be possession. That is what Satan does.
If that is the case, let me know.
Respectfully, it is not new and it is certainly not one size fits all.
Bipolar is a very real mental illness that can have multiple causes. The MRI of a person with Bipolar tells the story. You see a profusion of blood flow in a manic phase and very, very little in a depressive phase. The person truly hurts. Medication and therapy can help to ease that suffering. Spirituality can be a part of that help as well and is critically important to healing.