I dunno.
It's probably because when I was 12, my best friend outta the blue said "We can't be friends anymore," with no justifiable reason and left me with no friends, but I think saying "I don't want to be your friend" is really harsh.
Hearing that is REALLY hurtful, regardless of whether or not its justified.
I probably would just try to talk to him, and say that it's annoying that he never helps in group projects and that even if he's okay with cheating, you're not, and it's not an issue you take lightly.
Bottom line, everyone deserves a second chance. JMO, though.