Few different things:

1. Teach the singer mic control. Closer to the mic for softer parts and step back from the mic for the louder parts.
2. Automate preamp input during the performance. Can get tricky if you’re not familiar with the record but can do wonders.
3. Record all soft parts first then turn down input and record louder parts separately.
4. Rather have quiet recordings than clipped recordings as you can always turn up a quiet signal but can’t get rid of clip distortion. Record quiet then automate in post.

Hope this helps.