Minecraft – How to purchase skins for Minecraft through the Xbox/Microsoft Store

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.

Supposedly prepaid Gold cards are region-free, although Microsoft controls this and their stance is:

Some prepaid codes can be redeemed only in the country or region in which they were purchased.

... which is extraordinarily vague, likely on purpose.

You might want to contact Xbox Live customer support before spending any money - it's generally very hard to get your money back from these prepaid cards. Also, counterfeits and completely fake codes are extremely common from less reputable sellers, so beware any crazy deals or promises from vendors.