You have come across a classic double negative. Your misunderstanding is why people often suggest not to use double negatives in English (although sometimes they are useful). Normally someone would reply "I'm never not ready". It's a fairly common, albeit confusing, expression. It means "I am always ready". The never negates the not in not ready and essentially changes the "never not" to "always".

He's ready. I'd interpret it as an abbreviated form of "I'm never not ready". The pronoun, the verb 'to be' and the complement 'ready' are all in the question, so there is no need to repeat them in the answer. He only needs to add the "new" information: 'never not'. This double negative means that he is ALWAYS ready. In other words, he's ready now, as he always is. Note that this is not a common response to a question. I wouldn't recommend you use it.

I can understand the confusion! The meaning here is probably "(I'm) never not (ready)." - in other words, I'm always ready - but it's been abbreviated to "never not".