Its probably the plug cup then, same happened on my bike but it was intermittent would rev out some times but not others but it was on a 125 But tis a cheep and easy thing to check + if its not that then you have got a spare one.
yes gassers r the same even when there new there is still a bit of play in the bearings, its realy stupid they should develop some thing to stop the water getting in/grease getting out then it would be so much better
he can only ride a 125 at any events Some club may allow a 250 but i doubt it. the scorpa would be fine 4 him to ride it would be a bit heavey to begin with tho and the little seat "thing" dosent matter look at the old bikes.