Go gentle on your son - I'm 57 years old. First, I just bought my son a 1994 318i with automatic transmission. But with his car, you can press a "manual" button on the cars stays locked in the chosen gear. I basically did the same thing with his car as your son did to yours. Somehow, and I'm still not certain what I did as I wasn't paying attention, but I thought I was in fourth dropping to third, when in fact I was in third dropping to second. When I downshifted manually to second, I really red lined the car. Needless to say I was worried, that was about 4 weeks ago, so far no problems.
The reason you don't want to be too hard on your son is that you could do the same thing. I'm willing to bet your son worships the car and it really bothers him that he did what he did, with or without your added anger. No good kid sets out to destroy a good engine; they may do it out of stupidity or as in your son's case an unfortunate mistake. I'm willing to bet he won't do that mistake again, probably new ones... lol!
When I was seventeen (in the mid 60's ...groan) I had my fathers car, driving in the forbidden part of Madison Wisconsin - downtown (where he was fearful of the traffic I would encounter) and the automatic transmission just went on the car. Needless to say, I was upset by this incident - as well as spending several hours stranded - but I got the blame for something that was inherently a bad product - a transmission on a relatively new car. Even to this day, I am resentful of the anger my father had against me for an event that would have happened to him, instead of me, if he had happened to be driving at the "break point" time.
If you have a good kid and I suspect you do, he already feels real bad. Take your kid for a burger and on the way back to the car, say: "You know when I was a teen first driving I did this stupid thing.....(name your stupid thing... we all did it if we are male)... and now you've done a stupid thing to your car. Treat the car well, I know you love it - enough said."