Sorry to hear about your boy's knees.
I don't have any personal experience with this, but I have seen a lot of post on YT about it. I'm sure you can find some helpful info with a few simple searches. I'm sure the stairs would help a great deal. We have a step stool for Gracie to use to get up and down from the couch. Sure wish Boo would use it and save the wear and tear on his legs.