I'd discuss it with his doctor or find a specialist. Sometimes different medications work better for some people than others. Sleep disorders are not uncommon with people who have dementia. My LO was prescribed a small dose of Seroquel at night and she sleeps fine. She's not drowsy in the daytime either. Due to the changes with the brain and dementia, I might seek out a neurologist that regularly treats patients who have dementia.
Did you report the effects of the clonazepam to the sleep doctor? Perhaps a lower dose?