Please read this extract from the Posting Guidelines:
You are welcome to answer questions posted in the Ask a Teacher forum as long as your suggestions, help, and advice reflect a good understanding of the English language. If you are not a teacher, you will need to state that clearly in your post.
Answer a) is the standard tag question. Is he not is merely an alternative to a). You could use b) as it stands if you want to express a degree of disbelief- if the person were actually quite tall, for instance, then asking the question would the positive tag to show surprise/disbelief would work after someone had said that he was too short.