F16F22, loks like you figured out the answer by now - but some other things I did to get my son setup when he was starting out:
1. you can mix & match individual cogs from your adult cassettes to (for eg) create a 12-28, even if you end up with two 13T cogs. He can't use them anyway but it will allow you to pass rollout. (note that you can't use blocked gears at Nationals)
2. Use a 45T chainring - this passes with a 12T cassette and means you can use a standard wheel in case of a wheel change. However it's the least efficient/highest friction setup.
3. The best solution, if he continues, is to spring for the Shimano Ultegra 14T-28T in 10sp or 11sp and use a 52T chainring.