I have had no experience with Lewy Body disease but I can say that doctors do just guess at what treatment to use. There is very little research done on behavioral problems in any of the types of dementia. I do know that both of my parents (Dad with Vascular Dementia and Mom with Alzheimer's) have both been on Seroquel and Respiridone. I, like you, did not see much behavioral improvements with those particular medication but I do know people that they have helped. It is a hit and miss, try it and see if it works, type of doctoring. If you are not seeing improvement then you need to let the doctor know so he can try something different.
Be sure you are dealing with a doctor that is very familiar with the disease and how to treat it. We had no luck helping Mom until we found a Psychiatrist that specialized in treating dementia behavioral problem.
I did read a little on the Fentynal patch, which is for chronic pain management and I see why the side effects are scary. Yet I also know that those with dementia will "act out" when they are in pain. They do not know how to express that pain in any other way. It's truly a catch 22.
What types of behavior are you observing? Make a long list and talk to his doctor's once again for better options is the best advice I can give you