Supercat!
Seriously he ought to keep the cats in. Let them meow at the screen door to the balcony. If one jumps, such as at a bird, it is almost certainly curtains for the cat. Some kind of screening on the inside of the balcony bars would prevent their walking through, but it’s easy enough for them to jump to the balcony rail as shown.
There are commercial cat cages a person can buy that would give ample room for the cat to wander around within that would also give the cat a high perch to sit on and look around.