Cat food isn’t good for them but if it’s been a long time since he ate it, it has been converted to poop by now. The nutrition in the cat food has probably not been of any value whatsoever to him, because a cat’s nutritional needs and a rabbit’s are very, very different. It is potentially a health-hazard, so please, put the cat-food where your bunny absolutely can’t get to it!
Cabbage can be quite ok for bunnies, my bunnies eat cabbage almost every day. I read it’s a myth that white cabbage causes gas i bunnies, it’s a misunderstanding due to the fact that it causes gas in humans. But humans and rabbits are different digestion-wise.
Lots of hay is the best thing for a bunny-tummy and can compensate for bad food to some extent. As a rule, rabbits have sensitive stomachs, but some rabbits have “sturdier” stomachs than others. I have one who can eat just about anything and one who can’t have anything sweet or his tummy flips out.
Some bunnies have amazing capabilities when it comes to get into small spaces and high places and places where there is any type of edible stuff. I have one bunny like that, luckily he’s the one with the tough stomach. Maybe it’s connected, an adventurous mind and a sturdy stomach =)