Our kids were 10 and 8 when we went. Each kid had $20 for onboard spending. The deal was Mom and Dad bought photos plus 1 shirt of their choice (either on ship or at
Castaway Cay). We didn't do soda cards (we brought out own 12 pack - my kids don't drink much soda - in our house we have soda with pizza, that's it). The $20 could be spent on game cards in the arcade, souveniers, smoothies, etc. We did the purchasing on the islands. The kids spent that amount, no problem!
The kids will be 12 and 10 on our next cruise - we will probably up the amount to $40 - $50 and that will include their island spending too. Mom and Dad always have discretion to raise the amount or buy that "special" item if necessary (for exmple, we bought a model of the ship for my DS and the captain signed it - watch the store for the Captain's signing time and be early with your item ready to be signed!).
I think $100 will be fine - you may want to define in advance what you will/will not pay for and what he is expected to pay for during the 7 days.
Also, remember that this ship is cashless, so the key to the world card is what he will use for charging. You can check at customer services for an itemized list of what has been charged on what key - that may be worth checking frequently during the cruise so DS knows how much is left....
