I have of this alot. But my guy never had it but I have seen kids on the same bike on the same track crash from it.
First check the steering head bearings. Grease them and set them a tad snug. Raising the tubes can make bike unstable at speed. Put them back to stock.
Consult the tuner. Not knowing the spring rate or your sons weight or ability, it is tough to judge things. On pavement if you got that thing shaking then it could be the steering head bearings but a rippled road surface can cause havoc.
Good luck ... if stuck give some more detail on the fork set up and rider details.