You might find it handy for you to research a few things, so you know what they are when your DS comes home and talks about them.

I went to a catholic high school, and before I started mum taught me
the sign of the cross

Our Father (a prayer)
(Our Father, Who art in Heaven, hallowed be Thy name; Thy Kingdom come, Thy will be done on earth as it is in Heaven. Give us this day our daily bread; and forgive us our trespasses as we forgive those who trespass against us; and lead us not into temptation, but deliver us from evil. Amen.)

The Hail Mary (a prayer)
(Hail Mary, full of grace, the Lord is with thee, blessed art thou amongst women and blessed is the fruit of thy womb, Jesus. Holy Mary Mother of God, pray for us sinners now and at the hour of our death. Amen.)

I think I got through 6 years of catholic education just with those 3 and watching everyone what everyone else did.

Kids might also be doing their first holy communion, and their confirmation (grade 6 ish).

Some bible stories can be really strange if you haven't heard them before, even more if you are getting them interpreted by a 5 year old, but there is always google if you can't work it out.