As a conservative (at least in the liturgical sense) Catholic, I have to say I am a big fan of Bart Ehrman. I’ve read 4-5 of his books, though not the one you mention. But he basically says the same thing different ways, so I don’t think that’s a problem.
Bart presents evidence of things like textual differences, additions, subtractions, etc. and also contradictions (different genealogies for Jesus, different accounts of the events on Easter Sunday, etc.). Bart (and apparently you and your son) gets all upset by all this, and it’s one of the main reasons Bart became an atheist.
I think the correct response to all this is what Garry Wills says (“What the Gospels Meant”). The response? “So what?” If you’ve read Bart, you realize he’s constantly quoting and referring to Raymond Brown (Catholic priest and Biblical scholar) and Walter Ong (Jesuit). Bart does this so much that I personally think Bart would make a good Catholic. The point is that Bart, Raymond Brown, me, your son, the professor in Austin, etc. see the exact same evidence. We agree on the evidence. But our conclusions are different. If you believe that the Bible must be the word-for-word literal “Word of God” than you have a problem. If you believe–as Catholics should–that, as Dei Verbum says, the Bible is “the truth which God wanted put into sacred writings for the sake our our salvation,” you don’t have a problem at all. In other words, for example, what “truth” were the four Gospels trying to convey about Easter morning? Who arrived at the tomb first? Who appeared to the first to arrive? What these people had for breakfast or what clothes they were wearing? Nope. All irrelevant. The “truth” is simply that Jesus rose from the dead, and not just symbolically or spiritually, but bodily. The other stuff fills out a nice story, but as with any account of a traffic accident, different witnesses remember different details and they remember things differently. That doesn’t mean the accident didn’t take place. And, as a matter of fact, if police find that four different witnesses are telling the exact same story, they assume those witnesses are colluding. It’s when they differ in the details that the police assume they’re telling the truth. Likewise with the Gospels. So the details are different–so what?
As far as textual errors, additions, subtractions, etc. you have to look at the bigger picture. Which came first, the Church or the New Testament? Clearly, the Church–at Pentecost. The first books of the NT were the epistles of Paul, about 20 years later. So it’s the Church that validates and interprets the NT, not the other way around. And one key principle is that there is a coherent overall message in the NT; so any book that contradicts that message can’t be true revelation (so much for the Gnostic Gospels). So if the early Church accepted a certain verse that turns out to be a copyists addition or gloss, it’s accepted. If we were editing texts today we might leave that verse out–as Protestant scholars have done with many verses. But that misses the point: What is “the truth”? Are we restricted to a strict textual definition? No.
I think if your son understands all that, he will be just fine.