My chin keeps hurting his legs.

Hi guys. I don't regularly post here but I do read other topics.
My chinchilla Pudding is 2 years old now and seems to have a recurring issue. He keeps on hurting his back legs - first it was his right, then his right again and now his left. This has happened three times over the course of about a year. Each time I have taken him to the vet who has felt him over and said there are no fractures or breaks and that it is just bruising. He never goes off his food, just becomes a bit resigned and doesn't want to move and limps when he does. Each time it happens in the same way; he tries to jump up onto something, his front legs make it but his back legs don't and he falls back down. Each time the vet tells me to give him Metacam and after doing this for around a week he will be back to normal again, no leg issues.
I guess the reason why I'm asking this is because it is a recurring problem that I just can't figure out how to fix. The things that he is falling from are things that 95% of the time he can get onto just fine. It has happened again tonight and once again he is refusing to move around. He is still eating etc and I have felt the leg (which he doesn't seem to mind) and of course I will be taking him to the vet again. I am just wondering if there is any easy way to tell how serious these injuries are and if there is any reasonable way I can stop them from happening?