A small, short hospital cage is just fine for a chameleon with MBD as long as you can control temperatures well. You want to limit the areas he can climb because he is more prone to falling. Once he recovers then he will of course need to be moved to a more appropriate setup. But right now what he is in is fine for what's going on.
I also would not recommend cat food. As has been said, it is very high is protein, and thus can compromise the kidneys, which are already under some stress from the MBD. It is okay (but not ideal) for a few days just to get some nutrients in but after that I would switch to either blended crickets or something like omnivore critical care. You could even blend up a really good mix of high calcium vegetables and use that until he starts eating on his own.
A few decades ago the suggestion for treating MBD iguanas (because there were a ton of them during that time as the new fad pet) was to use dog and cat food. It was a widespread suggestion because they recovered with the amount of calcium in the diet. Even the gurus were using it. The problem was a few down the road from that all those iguanas started dying of kidney failure due to the protein levels in those foods. Now it is no longer used for that reason. However in the reptile world some news disseminates slowly. The people who don't practice a lot of reptile medicine might not be up to date with everything that's changed because this is a field that can change very quickly. We learn more and more every day. Already something I was taught in school that alters how you give injectable medications (the renal portal system) has been debunked, and the gurus don't even consider it anymore. This knowledge literally reversed itself in a matter of months all of a sudden but is still spread widely because most reptile vets don't even know it yet.
