Thanks for your question. The way that different people may respond to your question may depend upon different belief systems - those who belief in ghosts and demons may advise you to find someone to cleanse your son, and to some degree, whether you believe in this or not, it can help because it gives a good reason to believe that someone has bothered and has taken the time to get rid of it.
However, my approach would be different. Your son has obviously been assessed and diagnosed with bipolar disorder - to answer your question directly - the medications meant for this illness should bring about some relief, but if your son doesn't like them, it might be worth trying a complimentary therapy. I am going to suggest to you that you find an EMDR therapist (Eye Movement Desensitisation and Re-Processing) which can be used very successfully with those diagnosed with bipolar disorder, and which aims to break down beliefs about the self so that they go away. Have a look on www.emdr.com for more information and for a therapist near to you and your son. I think it would be wise to explain a little about your son's belief with the therapist before you book, perhaps on the telephone - find one who you feel understands you and your son, who you feel comfortable with. There are no guarantees with any therapy, but I feel this would be a good option for you both. Happy to respond to your reply, if you wish. i wish you both well.