Depends on how old your son is....
From the VS website:
A flying club family account allows up to three members of your household aged between 2 and 17 years to earn miles on qualifying Virgin Atlantic flights. Any miles they earn will be automatically transferred into your flying club gold account, so that you can spend them on whatever you wish. To request an application form, please call your helpline.
They upped the age as before it was kids upto 12 - as we all know that at age 12, kids become adults for airline ticket purposes. And at Disney, kids become adults at a younger age ?????
You can apply for a supplementary silver account for someone aged 18 years or over sharing your home address. Whilst they enjoy the benefits of silver membership, you will be able to spend miles from their account in order to reach rewards more quickly. To request an application form, please call your helpline.
Please note that both of these benefits are only availabe to Flying Club Gold members.
Thanks,
Richard