I think there’s a couple of positive things you can do. You may have to first do a little damage control on your son’s original blanket statement about the stories in the Bible in may or not being true. While the earlier (pre-Abraham, Genesis 12) stories can be looked at as literary devices that are nonetheless meant to impart real truths, the rest of the Old Testament is meant for us to understand as salvation history, and that’s the way we should take it. While it is true that history as the ancients understood it was not written with the supposed accuracy that we write history these days (that’s a complex subject that they can learn in college), it is still presented to us as history and should be taught and understood as such.
Secondly, you can tell them that the OT is important because it is God’s inspired word to us. He gave it to us for a some good reasons, namely, to reveal himself to us in time as the one, true God. He did this through the chosen people, the Jews, so they could reveal him to the rest of the world-- which means us. Also, the OT is a preparation, a groundwork laid, for the coming of the Savior of the world, Jesus Christ. It is pretty much impossible to fully understand Jesus-- who he was, what he came to do-- unless you have a decent familiarity with the Old Testament. One way you can demonstrate this is to have them look at their NT’s and note the number of OT citations (either quoted or in the footnotes).
Another way to show them this is to show them some clear examples of how Jesus was foreshadowed in the OT by various people, places and events. These are called “types” and if you can show them some of these, you can really make both the OT and NT come alive for them.
In Matthew’s Gospel, for example, you can pick out many parallels between Moses and Jesus. In 1 Corinthians, St. Paul compares Jesus, the Savior, the New Adam, with the old Adam who lost our justification before God. And Jesus’ multiplication of the loaves was foreshadowed by a similar miracle by the great prophet Elisha in the OT, except Jesus’ miracle is so much more amazing than it’s OT type, that it proves how much greater Jesus is than anyone that ever came before him. There are many more other types that can be pointed out.
Hopefully taking this approach will help you with your students. Next time you guys teach this class, be sure to study up on the OT. Next time you’ll be ready!
Hope that helps.