Weird!
Don't cull him. He should do perfectly fine. Sometimes animals grow parts that aren't really supposed to be there. I've seen pictures of an adult, wild, perfectly healthy-looking mockingbird with an entire leg+foot growing out its back.
I'd say trim some of the feathers away to get a good look at what's going on, but, unless he shows any signs of being in pain, no need to worry about him. He should be perfectly fine. And I want more pics, especially as he grows up!
There are two possibilities here; either something went wrong as his cells were differentiating as an embryo, or he absorbed a twin in the egg. If it's the first, he hopefully won't have any trouble at all, it's just that foot sticking out. If he absorbed a twin, he may have some issues from whatever parts of said twin might be inside his body. Given the location, I'd assume they would be in his brain. If he's acting normal, it's probably safe to say that he doesn't have anything else internally going on.
