I am very sad to hear about what your son is experiencing at school. As a mom, I send you my sincere hopes and prayers that this issue will be addressed for the sake of your son, and for all the others who are involved in any way.
All too often these things go unaddressed because no one speaks up. So, good for you for looking out for your child.

When I was employed in a school setting, I was told, too often in bullying situations, to keep notes/documentation. However it did little good because 'we had to wait' until something happened. From what you are telling here, this seems to have gone on too long already.
I suggest that you take all documentation with you (reports to school, reports to teachers, and child self reports, etc.) Also, enter this meeting in a spirit on non confrontation you can get more accomplished. However, "if" the other child's parent or guardian does show up, be prepared for a possible verbal confrontation and denial.
Who has arranged the meeting? Who will be participating? Are any other children being bullied?