cockatiel falling off perch when sleeping

by Carrie
(queensland Australia)

My 15year old cockatiel has started occasionally falling off his perch when he is sleeping. Once or twice a day. He then seems to have difficulty using his right knee for a while. I thought at first he was seeing wild birds flying around outside the house but it happened tonight after we had covered his cage up.

We decided to remove the top perches so he only falls about 6 or 8 inches. I wonder if he has something wrong with the right leg so that it won't lock when he relaxes. How can I find out? C