That depends on how it was timed. Assuming that it was standing start from first movement he should be in the low 22s range as long as his last 50m is not terribly bad.
I don’t think a 15 year-old needs to be trained specifically for the last 50m of the 200m. If you carefully and gradually build up his speed and fitness, things should take care of themselves over time. Too much SE is not good at such a young age.