Ugh, I really don't want to form a relationship with my mom's new boyfriend.
Uh-oh, is there a reason you don't want to have a relationship with him?
I've known him since I was a kid and there's always been something I don't like about him.
Perhaps you could try to get to know him better.  He's important to your mom so you'll want him to feel welcome as part of the family for at least her sake.  Otherwise, it could be put a strain on your relationship with your mom.
He's annoying and I don't like being around him.  I just want my mom to go back to her ex-husband.
Just give it a chance.  Maybe you have things in common that you weren't aware of and can bond that way.