Xbox – Am I going to lose access to cloud storage when the gold membership expires

Yes, they do. You can lock the lower price so when it auto-renews you save $20.

See here

One year subscription at $39.99 for the first year with auto-renewal at the then current annual price following year one

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.