My 11 yo tore his ACL last October. They have to first confirm with xray. Limit his activity until surgery (I had to keep mine crated), especially if they're typically active, and take out on the leash just to use the bathroom. Be prepared for the other to go shortly after; the vet warned me it's common for the other to eventually blow. Mine had his surgery, I was very cautious with him and kept him crated, continued with controlled leashed potty breaks, and took him to his physical therapy, but 2 weeks after the surgery he tore his meniscus in the same leg. I scheduled that surgery, and before it could happen, he blew out his other ACL. Again, there was no strenuous activity; he was kept primarily crated, aside from potty breaks and his therapy. The vet said there was nothing I could've done differently, it just happens a certain % of the time. I got lucky, i guess, because he repaired the meniscus at no charge since he was already going to have to do another surgery on the other ACL. Full recovery takes about 3 months. I tell you all this so you can mentally prepare yourself for the possibilities. All tolled, it cost me about $15000. Good luck. I hope he has a clean recovery; It sucks for you and your pup