What tyre size? If you’re going to Minet Park on Sunday then there’s gear inch marks set into the ground. You could then just wind down the upper limit screw on the rear mech to restrict his gears to suit the restriction.
He is on 700c x 23. He is 12 so technically an U14 but racing in the youth cat, I could’nt see any further restrictions for U14. The lads busy training and trying to get to grips with a road bike so wont be doing another CX race until the Ickneild one.