Skylanders Giants 3DS portal compatibility with 2DS

Yes, the figures are cross-platform compatible.

Make sure you buy your grandson a Starter Pack for his specific platform (either Skylanders Spyro's Adventure or Giants), and not a Booster Pack, both kinds include them game and figure(s) but only the starter pack includes the required portal of power.

The Spyro figurine isn't available for purchase by itself, the regular one was bundled with the console starter packs and Dark variant was bundled with the 3DS starter pack, but the Legendary variant is available in a Toys "R" Us exclusive bundle. Some figures that were only available in level packs or bundles for the first game are now available separately, so Spyro might be available on its own soon.

According to the Skylanders website, Skylanders Universe (the online app) is no longer available. The minigames that were available within the site are now accessible without an account and online codes are no longer needed or used for them, and neither a portal.

The online codes are now used exclusively with the mobile (iOS) games: Battlegrounds, Sky Patrol & Lost Islands. These don't require a portal either.

A portal is only required for the console version of the games and is available with the starter packs. There is a portal available for the mobile games but it isn't required to play them.