I know all about dislocated knee caps and they do hurt, they do not heal easily and even if you are lucky and pop it back, it takes a long time to heal. Read as 3-4 months of rigid brace and 4X weekly physical therapy. First time I did not listen. Second time was much worse and I listened for a while. Third time, took 3 weeks to reduce the swelling to the point where it could be popped back into place followed by 5 months straight leg and 4x week visit for therapy. I waited that time until both the doctor and therapist said I was good to go.
That was ~30 years ago and I'm sure medicine has progressed since then but I still do my excercises religiously and have not had a problem. Get him a couple bottles of bourbon, a bucket of ice cubes a picher of cold water and make him happy. Then it wont matter if he hurts.
Hope the docs can get him fixed up fairly quick. I do recall that icing the knee really helped. bag of frozen peas, towel on knee, ice pack for 15 minutes, repeat every hour if my memory is correct. Good luck and best wishes for a speedy recovery.

