Please help Mojo get surgery

Our beloved family Mastiff, Mojo, is in desperate need of knee surgery. He has a torn ligament to his left rear leg and can barley walk and without the surgery runs a high risk of tearing his meniscus if it is not done soon. We got him at 9 weeks old, he is now 5 1/2 years with many more to go. He is a beloved member of our family and loves to run and play and go for walks, playing tug of war is his favorite, which he can no longer do any of. My wife is diabled and home all day and he is not only her companion while I am working but also her protector. He is a big lovable 200lb teddy bear and very loyal and protective. He is so smart and so much fun but is in so much pain and so sad he is not himself anymore. It is heart breaking for our family, please help. It is beyond our means, any help would be so greatly appreciated - We met with the surgeon, his estimate to us is between $5,764 up to $7,444.