I would send an email to the principal and the teacher involved indicating the issue you're experiencing, and ask for the children to be separated at lunch time. There shouldn't be a problem with ensuring that these children aren't immediately next to each other. The major problem with implementation would be that the teacher usually isn't there supervising the children during lunch... she's in the teacher's lounge with her own lunch. Still, depending on how they handle the lunch room, there can still be things that she can do. For instance, if everyone lines up and eats in the same room, she can send the bully to the back of the line. I'd let the teacher and principal brainstorm about ways they can solve that, since it's their responsibility. I'd be letting them know, too, that I'm holding them responsible for solving it in a way that meets your child's needs without making him feel punished in any way.
Most school districts have written policies that describe how they take bullying situations seriously, and having sent it in writing means you can produce the notification later. That tends to help these things get taken more seriously.
My DD9 was experiencing a bullying issue earlier this year. In her case it involved three teachers, because DD is in a 3rd grade homeroom, a G/T math pull-out, and a G/T LA pull-out, all with the child involved. When I notified the school via email, all of the staff members involved took the situation seriously, took appropriate steps, and while the kids are still around each other all day and are clearly not friends, the bullying aspect has been resolved.
