Are luxating patellas a common problem in Cotons?

Hi, my Coton and I were doing agility when he started limping. After taking him to the vet, we discovered he has luxating patellas in his back knees. Now that I know about this, I often see him favoring his right back leg over his left, which seems to cause him more problems.

We've stopped doing agility because I don't want to degrade his knees any further and cause him to be in pain or need surgery. I have yet to invest in doggie stairs so he still jumps on and off the couch with no problem. He is almost 3 years old now but it seems as though he has lost all of the energy he had when he was a puppy (maybe it's because he's older?). Today was so nice, I took him out to run free and play fetch and all he did was lay around-he was not interested in running or fetching. I'm fearful he's becoming so inactive because his knees bother him. My vet has told me not to worry about it as his knees have not degraded any further and his knees aren't slipping out like they were in agility. I have started to give him glucosamine which seems to have helped but I am concerned about his future-he's only 3 years old, and I want him to be comfortable the rest of his life. I'm curious if anyone else has experienced this problem with their Cotons? Does anyone have any recommendations/suggestions?