Rimadyl is a good pain medication. It should help him a little bit. There can be a lot of variation in how you treat a specific fracture based on what the x-rays show. The treatment, general breaks down to a few components:
1. Pain Control
-Rimadyl is a good drug. It is a non-steroidal anti-inflammatory that will help with pain and swelling.
-I am not sure what the second medication is but many vets often use Rimadyl and a drug called Tramadol together. This combination of medications works well to control pain.
-If you think this guy is still too painful, even on his current meds, you can discuss a Duragesic or Fentanyl patch with the vet. These are morphine type medications that are in a skin patch. The patch is placed on the dog's skin and a steady release of pain meds occurs over 5 days. I use these quite a bit, especially in dogs with fractures. They can be used with the Rimadyl but not the Tramadol. It sounds like this guy could be a good candidate for such a patch.
*please DO NOT give him any over the counter pain medications (Aspirin, Tylenol, Advil)
-For a fracture to heal, the broken fragments should have as little motion as possible. How this is achieved depends on how displaced (near or far) the broken pieces are from each other. This should be evident on the dog's x-rays. Do you know if the vet recommended surgery or not? With pelvic fractures in particular, there are a lot of cases that can heal with just cage restriction. It can take a long time and require a lot of nursing care but many of these cases heal and do just fine. The pelvic fractures that heal well without surgery usually have minimal displacement of fragments, do not involve the hip joint surfaces themselves (the moving parts of the hips) and the bone fragments do not appear to block the pelvic inlet (this can result in difficulty urinating or defecating. So to be clear, surgery is recommended if there is displacement, joint involvement or pressure on colon or urethra from fragments. If none of this occurred the dog has a good shot at healing with rest.
-if surgery is not required 4-6 weeks of CAGE RESTRICTION will be adequate for a lot of dogs.
3. Nursing Care
-If you are lucky enough that the fractures do not need surgery, then the dog needs to be confined to a small area (extra-large kennel, mud room etc.) for 4-6 weeks. Follow-up x-rays should be taken at 4 weeks to confirm healing.
-During this cage restriction period-you may need to carry him out to the bathroom and even support him has he tries to go. Over time he will start supporting more and more of his own weight. It sounds like you are doing the right things. A sling is a great idea
-Keeping them clean-these guys are painful and have a difficult time urinating. Use "pee pads" or puppy training pads and place them around him in his kennel. If he has an accident these will help to absorb the urine. Frequent baths will help prevent urine burns on his skin.
-In some cases, I have hospitalized a few of these dogs and placed a urinary catheter so that they don't have to get up and go out. It all depends on how painful they are.
-During the initial few days you may need to hand feed him and deliver his water to him. They are very painful and don't want to move. They often want to eat but don't because it’s too painful for them to move.
As you can see there is a lot to consider. I would start by finding more out about the x-rays. It may be this guy needs surgery. If surgery isn't recommended, then the principles above apply and his chances at recovery are good. It’s a lot of work and can be difficult to watch sometimes but they usually show improvement every day.