Splatoon is only available on the WiiU, and it is a blast to play.

As for the 2DS, if you're ok with the form factor, the 2DS will play all current 3DS games (just without the 3D effects, which is probably best for your son's young eyes).

It may be worth mentioning that there is a "new" 3DS being released that is slightly more powerful. Most games will still be made and targeted at the old 3DS/2DS, but games specifically designed for the new 3DS will not play on the 2DS (Xenoblade, for example).