My baby girl, Kaya - 3 yr old brindle, completely tore her cruciate ligament when she was just 2. The post op recovery (not to mention the cost) was the worst experience of my life. She was in so much pain, she yelped everytime she had to get up and down off her bed. Her leg was completely casted, and we had to hold her up to just pee. Saddest thing to see her hurting like that. She held her leg up, as you described your dog did, off and on for 8 months or so before she did the final blow out. I wish I had known then what was going on so we could have prevented it. I just assumed she had played too hard when she limped on it. The continual fraying of the ligament without time to rest eventually did it.
About $2,000 in vet bills later, and several months of post op care, and thankfully she is back to chasing squirrels and cars, and harrassing her big brother.
As someone else posted, it is likely for the other knee to go too, so we are keeping a close watch. She limped on her other knee the other day, and we restricted her to the leash for about 1 week after the limping stopped. I am not taking any chances!
We have one of those stakes that you can get at any pet store that has a lead on it. She loves to be outside so this was a life saver during her post op. We still use it when we see her limping (still occassionally happens on her knee after the surgery). I highly recommend you invest in one and keep him restricted so the injury can heal - whatever it is. Also, if he overweight that can be a cause as well.
Good luck!