You also mentioned that he has a number of psychiatric diagnoses. The fact that he is currently not on medication may mean that he is symptomatic. Those symptoms could potentially be causing his sleep problems. Again, it is difficult to pinpoint the exact cause of the problem. There are many variables.
I would recommend that your boyfriend be evaluated by a physician. The physician can refer him to a local sleep disorder clinic. If your boyfriend has health insurance, that is a good place to start. He should also contact a psychiatrist for a thorough psychiatric evaluation.
It is also concerning that he has stopped taking his medication without supervision. I understand medications can have unpleasant side effects but it is not wise to abruptly stop taking psychiatric medications. Your boyfriend should have spoken to his psychiatrist or his prescribing physician about this before stopping the medication. The psychiatrist or physician could have advised him about the safest way to discontinue his medication.
The bottom line is that there is no easy answer to this question. Your boyfriend needs to be evaluated by medical and psychiatric professionals to determine what may be contributing to his sleep problems. Encourage him to be evaluated as soon as possible. Offer your support during this process. It can only help. I wish you well. Please take care.