I'm speaking from personal experience. I work at a state university that unfortunately has had this happen. It wasn't in my building, but at the time it was thought the shooter had entered where I work.
This stuff was not part of the world I grew up it. It didn't happen. We were clueless how to act, where to go, what to do, etc. We drew the drapes and hid behind cement pillars (those who could.) Some of us did pretty good, stayed level headed, and were able to listen to directions. Others completely melted down and had to be physically pushed in the directions we needed to go. Being escorted out of our building by swat members left some people frozen in place, while some broke into sprints, just rushing away.
When I think back on the time I realize how deep in shock we all were. There is no good way to cover all the basis on what might happen, but you do react better when you aren't so stunned.
I think what would be good for your son to ask is "what should I have done?" I would suggest explaining the classroom situation, how everyone reacted, and how he reacted and ask what they suggest. I would think if he was asking this as a serious, legitimate question he should be addressed because that's what matters -- what he should have done.
In the days that followed what happened to us we did ask that ... what should we have done? We have a ton of store rooms and places to hide, yet we were all gathered in a rather open space surrounded by glass, attempting to hid behind pillars. We were literally clueless, sitting ducks.
I know we all have our own plans of actions now, but sadly we learned from real life. Is it likely to happen again? Odds are no, but I would have said that if you'd asked me if it would have happened in the first place.