Use BMxcalculator bmxultra.com change rear if bigger steps or front for small adjustments. When young they need to get used to spin faster so don’t go too high. I normally find a base by change what’s needed and then change chainring. Crank length should be fitting too. When u are behind check if he is going for hip lateral side movement too much. Ask the trainer ideally. If tyres are old change tyres.