Sold 'As Is' means nothing.
You have to use "broken- for parts" if you don't want to take responsibility for the function of the widget.
Your response sounds fine. If he comes back at you try "I regret your purchase was not to your satisfaction. Please return it for a full refund."
If he asks for a partial refund'-- "I regret your purchase was not to your satisfaction. Please return it for a full refund."
If he threatens negative feedback'- "I regret your purchase was not to your satisfaction. Please return it for a full refund." (and Report him for feedback extortion).
While phones are a high fraud item, most people are honest. Even the cack-handed and stupid ones.