Overall I agree, the OP wasn't in the wrong and should celebrate his son's first birdie. That said, I could imagine a thread from just about anyone here that reads like the following;
"I was on track to shoot my lowest score, when right in the middle of my swing off the tee a loud cheer erupted on the green behind us and it caused me slice my drive right into the water. I was still annoyed after my penalty shot and drop and proceeded to double bogey the hole, thus messing up my chance to break my lowest score. When I saw the group, I was pissed and yelled at them to shut up."
Admittedly a bit of a devils advocate play here, but I think if a thread was started in this way, the responses might be a bit different than what we're seeing.
I don't completely agree with Fourputt, but his point is well taken, most of us don't want people yelling and screaming when we're in the middle of our swing. Does it happen, of course, do I think the OP did anything wrong, not at all, but let's not be so quick to make a judgment that no one would ever be annoyed if a round was ruined because someone yelled during their swing.
Congrats to the OP's son, I didn't get my first birdie until I was 47 so he's well ahead of me and Welcome to the site.