I would recommend that you request that the principal does NOT meet with your child and his bully together. Bullying involves an inequality of "power" in which the bully has some leverage over the victim, usually related to to age, size, etc. Bringing the children together for the sake of mediation is only effective when the children are on equal "footing." Given that this is not an equal relationship, this sort of meeting has not been found to be effective.
Instead, depending upon the school's policy on bullying (assuming they have one) should be enacted. Hopefully, it involves some sort of consequence for the bully. Teachers and school personnel (e.g., lunch ladies) should be made aware of the bullying and told where the bullying occurs. They can keep an eye out for the bullying and intervene immediately. In addition, your son should try and stick with a "buddy" throughout the day when the bullying most often occurs.
We have gathered together articles on bullying, written by the experts in the field just for parents of victims like yourself. See the URL links below.
L. Compian, Ph.D.
Education.com Expert Panel