Depending how close the contract is to renewal/upgrade, it may simply be a case of settling the outstanding device plan (if it's a refresh) and then doing a transfer of ownership on the number. Process of this, in simple terms, is that your Son takes out his own contract, either with a new handset if he fancies an upgrade or a SIM only if cost/credit status are an issue. You both then consent to the transfer, and he can keep his existing number on a new contract in his own name.
If it's not a refresh, you'd need to settle the remaining minimum term in full, so often not a cost effective solution.
Just a word of warning re billing address, I'd be very careful changing the billing address to his whilst the account remains in your name, as this could cause you potential issues in the future...If a "new" address is reported to the credit reference agencies then you may run in to problems when applying for credit products using your current ("old") address.
I work for O2, however any advice or opinion given in this community should be considered my own and not necessarily representative of Telefónica UK Limited.
