You may well have already considered this, but, is setting up an email account for your son as a secondary address to your account the best option.
It will always be tied to your account and will be lost should you ever cease to be a VM broadband customer.
Your son will never be able to 'own' the account. Unless you actually want that control over the account consider setting up an email address not linked to an ISP such as Gmail or MS outlook, others are available.
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