So many people telling you to find a new group! Screw that, there's actually a very easy and elegant solution.

Classic conditioning.

Otherwise known as Pavlovian conditioning, named after the Russian scientist who discovered that if you rang a bell and fed dogs at the same time, eventually they would begin to drool at the sound of the bell alone.

What you want to do is basically associate instances where he treats your character unfairly with unpleasant experiences (otherwise known as positive punishment). The trick is to make sure he doesn't actually notice that's what you're doing, but that it does have an effect.

So you can't just kick him in the shins or spit in his sandwich. The first is too overt and the second is too subtle. Instead you want to aim for a happy balance.

Try something like this: the next time he fudges a roll, casually say "I have the worst damn haemorrhoid right now! Every bowel movement I have is dark purple!" It's not the sort of thing that he'll notice as being punitive, but you've just made him subconsciously picture a giant infected boil on the inside of your anus.

If you do this enough he'll soon begin to associate fudging rolls with you saying gross stuff, and develop an aversion to it. It doesn't all have to be infected anuses or "accidental" reveals of pictures of Margaret Thatcher naked. You can also use positive reinforcement. Make some bomb ass brownies and whenever he lets you do something cool, offer him one. He'll associate saying yes with getting some chocolate fudgey goodness!

Soon you should notice a change in his behaviour. Just remember to nip any regressions in the bud. Good luck!