Very few of these characters translate to other fighting games, especially the beginner friendly ones.
As of now, Sackboy is the easiest to use, but he translates to no other fighting games.
Raiden is next easiest, but this is if you're used to memorizing simple move combinations and I can see beginners chocking and getting frustrated in the heat of combat. Raiden is ass if you can't finish any of his combos.
Ratchet is very beginner friendly, but he's kinda like Sackboy in that he translates to no other fighting games.
I say **** it. Just let him pick a character that "looks cool" and let him beat up on the computer and then try some other characters until he gets a character that feels like. --- Sanity is a one trick pony, all you get with it is rational thought, but with crazy the sky's the limit.