Update: My regular vet did some research (plus I told her what the breeder said) and she now thinks that it's likely that nothing is really wrong. He is just growing and the knee joint is a bit loose. Again, I'm very relieved to hear this, but I'm still concerned and keeping a VERY close eye on him for signs of pain. My vet recommends that we do x-rays again at 9-10 months to check the knee joint and make sure it still looks ok. I'll definitely do that.
I think I'm hearing the clicking less, but I'm not 100% sure. I used to hear it countless times a day, and now most of the time I hear it fewer than 10 times. But I'm not sure because of course I'm not listening to his knee all the time. Otherwise, he is still acting fine.
I had made an appointment with the orthopedic vet for later this month, but with this new info from the vet, I may cancel for now and see how Kip is doing in a few months.