Stop app charges appearing on mobile phone bill (UK)

My son has an 1phone 11 and I pay his O2 phone bill. Is there anyway I can prevent App Store charges/subscriptions being added to the mobile phone bill but rather come out of his Apple Account balance or his own debit card?

I don't want to prevent him being able to download new apps or needing my permission each time. I just don't want any charges being added to the bill I pay for

Thanks