Have a rational talk with your mother. Tell her that you don't like him and give her concrete reasons why this is so. Try your best to keep personal bias and emotion out of the conversation. If you sound calm and rational, you have a better chance of being listened to.

I agree with what's below but have to add: Then be prepared for your mom to still see the man. She obviously likes something about him and just because you've registered a complaint doesn't mean she will stop seeing him. Nor should it unless he's really up to something he shouldn't be.