How young can a child be to play the original PS2 Kingdom Hearts?

My son is 7 and I've been playing games with him since he could hold a controller, so he's very capable. Heck, one of his favorite games on the DS is N Plus, so he's got the hand eye coordination down. I was wondering how complicated the story is in Kingdom Hearts. Will he get lost and not know what to do next? He's buying himself a PS2 today and wanted me to get the game for him but with the game's Final Fantasy roots I was worried. If it helps, the games he's into at the moment are Fusion Fall, Skate 2, Madden and NBA 2k8, and Burnout Paradise. Should I get him Kingdom Hearts or look for something a little easier? I'll take suggestions on another game to get him too, something with a story he could follow, thanks.