No need to report it as it’s unlikely to enhance his application.
As an aside, the mean was 4.4, so it’s not awful. It is much easier to get a 5 on an AP exam than a 7 on an IB exam. (Only 3.9% got a 7 on that exam, and the score that was awarded most was 4.)
If your DS didn’t do as well as hoped on the other SL exams he took, he may want to work on prepping for his tests next year. It’s a very different assessment than the AP. If his school is an established IB school, his teachers should be able to help. Just mentioning that in the event that he was approaching this the same way he did the AP exams, where his approach was successful.