What size is that rig?
Where did you place the mast foot?
My experience suggests that for your son to be able to "steer" with the rig, the
mast foot needs to be absolutely as far back in the slot as possible.
I did some testing with 1.2 m2/1.7 m2 and 2.5 m2 rigs and had to make an aluminum
mast track extension to bring the mast foot back over the center fin.
Without the extension, the tiny rigs (< 2.0 m2) center of effort was simply too far forward to make any sort of progress upwind.
Hope this helps,