How do I get someone to find out they can't sing as well as they think?

I'm doing a cover of "What You Own" for a school performance and I was igned with someone to sing it with. I can sing higher than him but he wanted to take Roger's part (the higher tenor part) so I was like whatever but he cannot reach that high A so I intentionally taught him the melody during the rest of the song while I sung the higher harmony but I wanna switch to sing Roger's things because he cannot reach it! The highest he can sing is a D probably, not an A! Anyway, his diction is terrible and he's always flat when singing high notes. He thinks his notes are always perfect. How can I get him to hear himself and let him realize he cannot sing as well as he thinks so that he would BS when we practice?