He is the only decent person in his family.
It's solely one. The only one. The sole one. Totally unique.
The reading for 唯 is the same as the one you learned with the kanji, though there is also a shortened version which has ゆい shortened to ゆ. The reading for 一 is one you haven't learned yet, wow! You're finally going to learn the second, rare on'yomi reading for 一! It wants to be the only reading now, so it eats (いつ) the other one. Oh no, you can't remember that other reading now. いつ is the sole 一 reading in your brain!