I’m sorry your bun is going through this. His capacity to clean his rear end eat his cecotropes as they emerge from his anus is dimished due to his legs being bandaged. You will probably need to help him with his personal hygiene until the bandages can come off. As your vet says, sticky cecos are likely a result of changes to his eating habits and lack of exercise, plus stress and meds. You can try giving him a probiotic (for example benebac) to help his tummy. It’s very good that you’re giving him critical care, it’s very important that he gets all the nutrients his body needs to heal the broken bones.
Rabbit bones as a rule heal well, but since he is older, healing may need extra time.
The website Disabled Rabbits has many tips for how to best care for disabled rabbits. Your bun is hopefully only temporarily disabled, but many of the tips on the website could still be useful:
http://www.disabledrabbits.com/