a pw50 is a great bike for a beginner, but if he's plans on jumping i would go for something more modern as the suspension is crap
the front forks don't even contain oil, just a spring in a tube!
also drum brakes
my nephew started out on one but as soon as he started doing jumps we had to upgrade him to a ktm 50sx which has a lot more suspension travel, mono shock rear, more power and is the same weight
he has only just turned 5 also