The bit that intrigues me is your comment that your friend is a good singer. It sounds that he is demotivated and may be asking for your opinion to confirm his own. Have you asked him what he thinks?
It's difficult. If someone asks your opinion, it's disrespectful to be dishonest. That doesn't mean you have to be unpleasant about it, but criticism (in its literal sense) is always helpful. That is equally true when it's not entirely positive.