most scooters have an electric choke , so initial start up can be a bit dodgy , usually they go up to around 5000 rpm then after a few minutes it will settle down to 2000 depending on model
options :
you may need to clean the carb ie remove the air filter and when engine is running spray some in.
the fuel mixture is wrong , check your manual for correct position , usualy around 1 & 1/2 turns anticlockwise from frullin position .
air leak , if your son has tried to de-restrict it , ie remove air filters or one of the filters to the carb isnt on properly
check the above see how it goes
jase, January 2008