Suggest Pain Medications While On Gabapentin, Notriptyline And Tylenil

My husband is on home PD and has diabetes as well. His neuropathy is causing him so much pain and has worsened. He is currently taking Gabapentin, Notriptyline and Tylenil extra strength but doesn't seem to be working. His endocrinologist is not very sympathetic. What else can relieve his pain?