I dont think he did it to cause harm to anything or anyone and looking at a 5 year old's mindset it was just a quick action without much thought going into it. Kids are going to do things without thinking that has consequences like that.
I remember your son at Chris and Nina's this past May; I could tell he was your son because he looks so much like you :) and he was very polite and very well mannered. I wouldn't be too severe. Just hope he doesn't continue to do it; then you may have a problem.