XBOX Live Gold Sharing

I would suggest buying your son a prepaid card. You do have to remember when it runs out and buy another, but they tend to be $10-15 cheaper than buying the same amount of time through Microsoft directly. It's also much easier to stop being a subscriber, since you just don't add more time to the account - no need to figure out how to stop the automatic payments.

Yes, your Xbox Live membership travels with the account, not the console. So if you have a Gold Account signed into a console, you have Gold privileges on that console. You can recover that account on a different console and have Gold privileges.

This is also covered in the Gold Family Pack FAQ.

Q: I have multiple consoles in my family. Does my Xbox LIVE Gold Family Pack have to reside only one of those consoles?

A: When an individual is added to an Xbox LIVE Gold Family Pack, that person’s Xbox LIVE account must be on the same console as the primary account holder. However, once they are a part of your Xbox LIVE Gold Family Pack, that person could recover their account on a different console and still remain a part of the Family Pack, as long as they reside in the same country as the primary account holder.