Xbox – buy the Xbox Live Gold Family Pack for only 2 accounts

According to Microsoft, Yes.

To do so, you need to add the secondary accounts to the same console as the primary account holder. You then "activate" them into the Family Pack. After that, you can recover those accounts onto different consoles, and etc.

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.