he can use a spike chain, which would give him the 5 foot reach.

and standing up from prone does incur AoOs
From what I've been told on the boards, Reach doubles your Natural Reach - 0ft x 2 = 0ft.

And AoOs don't get sneak attack damage unless they're triggering sneak attack is also in effect, like being Invisible or whatever. Unless we're talking about the feat somewhere (I think it was) which gave you sneak attack on AoOs.