Thank you for your question.
If you are acting as a managing member in a single member LLC which you incorporated on his behalf, then you are the owner of the LLC.
Virginia law permits a minor to be an owner of an LLC. However, a minor cannot be bound by contract
and cannot sign a binding contract. That would mean that all business contracts would need to be signed by you until he is 18 years old.
All that you need to do is draft a Membership Agreement that grants him a membership interest in the LLC. If you stated in the Articles of Organization that the LLC may only have one member, you would need to amend. However, if you use the general forms available, you likely did not make this a term and are allowed to simply add a member through a private agreement.
You would use the following form agreement (making adjustments where necessary): http://www.lawmart.com/forms/bus-lc11.htm
In the spot where it is talking about how much you are selling the membership for, you will say "in exchange for (Son's Name)'s work and talent".