Yikes! I am sorry to hear about that. I am glad he is OK other than the scar. I can't answer authoritatively about the airbags not deploying. In my experience, sometimes they do for certain types of impacts, while others surprisingly do not. I think it has something to do with a thrust sensor deciding if it has enough force to deploy.
I will pass this along as something I heard from a veteran EMT and driving school instructor that has stuck with me.
Sun visors. They are either "fully deployed", as in touching the glass of the windshield, or "fully stowed", and nothing in between.
He described the type of injuries that something as innocuous as the sun visor, scalping drivers and breaking noses when they they are only partially pulled down and aimed at the face. Be careful and let your son know this.
I went though EMT school and I cringe whenever I see some passenger riding along with their feet on the dash, or cocked out the window. If a passenger side airbag deploys with your feet in the way, you might not be walking anytime soon. There are lot of bones to break in your feet.
Take care and be safe brother!