Jack and family welcome. Sorry you found yourself here but you are in good company. Many people with giant breed dogs are part of our community so you'll be hearing from them soon.

Meanwhile, about your question:

 My question is what quality of life would he have if we amputated and roaming around a large area is his nature and job?  

That is a tough situation but it's not a deal-breaker when it comes to ensuring a happy life on three legs. We've known many dogs who were in the same situation. Tank in Australia is the first one who comes to mind. He's not a giant breed, but he has a job like your Jack's. Also, Kylie Jean, a Cattle dog, is another. There have been many others.

You don't mention if Jack's front or rear right leg is affected (did I miss that in your post?). Although both types of amputees have challenges, a front legger does experience more challenges with movement. 

If Jack patrols independently, it will be up to you to watch his activity level so that he doesn't overdo things. Too much too soon, or later on, can lead to muscle pulls and soreness. If you have access to a canine rehabilitation therapist, regular check-ins can offset the stress his body would endure.

Lots of people here use dog strollers to help their dogs go on longer expeditions. Would this be an option for you and Jack? How big is he? We've seen strollers be used for Rotties, so pretty big ones area available.

Would a limb salvage procedure like this one be an option?

It sounds like Jack is very healthy and strong otherwise. He has every chance of bouncing back and having a good recovery, and enjoying life on three legs while fighting cancer. He may outlive the prognosis or he may not, but amputation can give him a pain-free life with you in the interim.