You can still take him outside. Get him a leash. Or build him an enclosure. Do you realize that with only three legs, your cat won't be able to run as fast or jump as high? He will adjust very well to the missing leg, I'm sure, but he won't be able to get away from dangers as easily as he had been in the past. If you really value your cat (and I'm assuming you do, because you are willing to get him the surgery and help him through the healing process), you will find a way to keep your cat SAFE. This is your one chance. Your cat has been hurt, yet he survived. The next time you will not be so lucky.