Hi Sam, Welcome to JustAnswer. I'm a licensed veterinarian and I'll be happy to help. Sorry to hear about Reno's discomfort. Maybe he does have fleas. About 50% of dogs are allergic to flea saliva, and even one flea can create severe discomfort. The other 50% of dogs aren't allergic. Is Reno on a good monthly flea preventative? If so what? Does he have any fur loss or skin lesions?
My other thought from what you are describing is that he has pain in his hips or lower back and it sitting unusually because of that. He is on an anti inflammatory, however, and that should begin to help him. Which one is he on and how much, and how much does he weigh?