I know as it relates to parakeets [hehehe] that birds can vomit, but they usually don't. If he did vomit, he would have bits of food all over himself. They can and do regurgitate at will, though. I also 99% remember reading/being told that animals in general are not as prone to getting food poisoning. Something about the way they're all set up. It happens, obviously, but it takes more food and much "less fresh" than it would for us, and usually it's not as serious a problem.
He's probably rejecting the cat kibble because it did make him sick to his stomach. At this point, his mum would be feeding him his staple diet, and he would be entrusting her not to feed him something bad. Mum would know not to eat certain things because of her mum, her mum's mum, so on and so forth. Animals are smart like that. One bad experience is all it can take, especially in wild animals. That's why national parks are so hard core about feeding the wild animals. Only takes one time for them to learn the habit. Anyroad, maybe you can try flavoring the cat kibble with something for a bit? Baby food or something, maybe? Or maybe even crushing up the kibble with the egg.. Wean him back to the kibble sort of thing.