I'm sorry that you received this news, but if Thor acts like it's not bothering him then that should bring you some comfort. My Izzy is on Cosequin for her LP and is still doing fine with just taking supplements, as opposed to surgery. I had to get a second opinion on hers because her breeder insisted and was told by both that hers are a grade 3. The first vet said surgery immediately and the second vet recommended that we try supplements and see how she does because she wasn't acting like it bothers her. The only thing Izzy will do is that she walks pretty straight legged....but not always. She is very active and probably more than she should be, but it doesn't seem to hurt her. She is our one that will jump up and down like she is on springs when she is excited to see us. We used to try to stop her, but she does it a lot and we can't always get to her right away to pick her up. Your Thor sounds like he is doing just fine so *try* not to worry (easier said than done, I know).
Hugs to you and that sweet boy!