VM Broadband/TV/phone accounts are completely separate from mobile accounts. They are billed for separately and managed separately et-cetera. So regardless of what happens to the mobile account, your broadband/TV/phone account won't be affected.
From what I've read, the mobile side has no transfer of responsibility option, so you couldn't transfer the mobile account over to your son. You could however do something like PAC code it to a PAYG SIM with another company, then from there you could move the number back to VM under his name or just PAC code it to another provider under a contract deal your son takes out with another company.
- - - - - - Any opinions expressed by myself are entirely my own and do not represent Virgin Media in any way.