Dont weaken his good side and give him a dark side to deal with.
That a double negative, no carrot. all stick, Unless you want to punish the player. It seems like a great way to alienate him
"Hey, your weaker then everyone else, and part evil. Enjoy."
dont compel the hell out of his aspect either, that's just cheesy. and will lose it's "Flavor and interestingness" quick. and by session 2 or 3 the eye rolling will start
Tempt the character and player, in roleplay. Give them little boons and on occasion larger bonus's, Think of it as compelling the players aspect's instead of the characters. If you be rude to this shop owner, next roll you get a +1. If you take the dark spellbook for yourself, You will be able to get the juice to break the curse on elza....(okay, bad examples, but you get the gist)
Dont make it a on off evil switch, Lead him bit by bit over to the darkside. Evil isnt just the big temptation's. it's the small ones too. remember that ^_^