Your player may need to win all the time, in which case I don't think that there is much hope for him.
If, however, he just hates to lose, there may be ways to deal with it.
In PFS, there are shirt (and other) re-rolls. You might want to go to a Hero Points system (APG).
The Weapon Master archetype gets a free attack (or confirm or damage) re-roll for every five levels she has.
A Bit of Luck from the Luck Domain is a great buff since it lets the player roll two D20s for the entire next round.
If you did all of the above, he might still want to cheat. So, you should have all of you players call out the rolls of adjacent players.
It might very well be too late for any of this to work.
(On the subject of losing: for chess players, the pain of losing is four or five times greater than the pleasure of winning. Sorry, I saw this a long time ago, so I don't have a citation.)