Many players have dice superstitions, In fact I've seen several debates over this on the general board in the short few months I've been here. I myself have some quirks which I use to roll stats, and while they have no physical results, they make me feel better. There is no reason why rolling one die to roll stats should have an adverse effect on the outcome of the player's stats. He just happened to assign them well. I don't think the solitary die is your problem.
Was this part of his backstory? Or was this something that you as the DM inserted into the game?
This was really something you should have dealt with before you started playing. However, with stat enhancing items, players tend to even out over the course of a game. Also, you will find that in the mid-higher levels, stats aren't the most important numerical aspect of the game.
From the information you have given, it doesn't seem that he is a power player. His character is apparently from a history that is unliked by the players, but that opens up some interesting R-O-L-Eplaying avenues.
If the character is blatently overpowered, don't kill him as an immediate fix-all, as a precontrived death could lead to problems with the player. You could insert items that would boost the other characters to an equivalent power level, or you could have your party attacked by creatures that do Stat or Level drain. Try to work with the strengths and weaknesses of the character, rather than attempting to erase the problem. Your player might make a character that is more annoying than this one.
However, if all else fails, the deck of many things is a nifty way to lose your soul, or be imprisoned in a far off plane, etc.
-G