Any one who remembers Jesiri last year will know he was a rescue cat with one broken leg (front) broken pelvis (back leg, which is why it was so bad, no stability on either front or back), broken tail and broken jaw..
With lots and lots and lots of TLC, vets, money and foster care, he recovered after 10wks in a kitten pen.. He ended up losing his tail but is now a full member of the fosterers home as they took him on after pulling him through..
Its not easy with two broken legs and the important thing is that they can still use the litter tray.. but i wouldnt give up at all.. It may cost a fortune but rest and time is whats is needed, if he is young them even more so...
Good luck..
BTW we are lucky to have a vet who specilises in bones etc locally to us.. You could ask your vet for a referral if the op is difficult.. He works in Faversham and is suppose to be one of the best in the country!