In an uncondtioned person this will be very common and a young persons heart can easily work at this pace. Also don't rely on heart rate monitors ( the kind used for working out) as they can give many false ratings.
The average max heart rates are 220-age. Your son's predicted max is 209. That is abot 86% max predicted. The time to reach a heart rate is not only dependent on the level of exertion, but also stress level, rest, and the time given to warm up. Unless there were symptoms of some sort, I wouldn't give it a second thought.
I hope this helps.