So if I could just ask, why are you neutering now anyway? Any particular reason outside of "popular opinion" (which these days as you can see, isn't that popular any longer)? Honestly, as long as you know you are capable of preventing "oops", he's not "off his rocker" from the hormones, and you aren't at the decision to rehome, what's the rush? I can see what you mean about putting them under multiple times, but in this case and any with anything related to structure and age I would wait as long as possible. Heck, I don't know about Kelpies but for the most part the average dog doesn't usually fully mature till 2-3 years old so he's still got a ways to go, and with a good physical exercise program he may never be 100% right structurally but can improve. Swimming should be excellent for him, you could also try working a lot with balance on an exercise ball every day, getting him to stretch and work the weak leg in a low impact manner.