I'm so sorry this happened! Is there a vet in your area who deals with tortoises? Or maybe a veterinary hospital? I think I remember seeing on TV that a turtle's shell was repaired using fiberglass or bondo - something like doing bodywork on a car. Without knowing much about the situation, I think it's at least worth a trip to the vet, if you are able to do that.
Well, if he's hung in there this long, I'd say he stands a chance, but I know nothing about tortoises. I think it's time to reach out to a local vet, or animal hospital - they can assess him & his injuries and would be the best ones to advise you of your options. I'm pretty sure holes in his shell can be patched, but I think it's best done by someone who knows what they are doing - the material would have to be sterile, so as not to cause infection, and you made need to give him antibiotics or pain medicine, too.