Actually, I think that's more complicated than at first glance: I suppose you could say better in English if you meant "better (at participating) in English class." The in would refer to the class, not the language. If we say better at English, it we're thinking of an activity, like better at speaking English. On the other hand, if the preposition refers to the book for example, we'd say "He's better at writing in English." If you then left out the "at writing," the elliptical sentence would be "He's better (at writing) in English. I hope that helps.

You would use in when you are referring to doing some other action through (or using) English. E.g. I am better at arguing in English. 'At' would be more common when you are talking about English directly. E.g. I am better at English than Korean. (you'll notice that the ' at' in the first sentence was applied to "arguing" rather than "English")

"At" is correct, since you're describing a person. Now, to describe a book, a sentence, or whatever, you would say "in:" "The movie is better in Dutch."