
This is the same situation I had with one of my Chows. As with your boy he had first had one leg go and a year or so later the second had to be done. Everything was fine for 7 months after the 2nd op but then very suddenly he was unable to use the leg. I think the naughty boy had jumped on the sofa when I popped out for an hour.
He was diagnosed with a meniscal tear which the specialist said was a complication that occurs in about 2% of dogs following TPLO surgery. It does require another operation but as Christine said the cost is about half that of the original surgery.
I really feel for you, it is so stressful (for you, the dogs cope so much better).
Sending healing thoughts
Mel