Hi C.,
Kids just break my heart. I feel badly for your son! Always good to let the teacher now, hopefully she will keep an eye out for trouble. I don't know if I'd go as far as the conference, not just yet anyway.
I would attack the problem from all angles. Telling your son to ignore them is first. If the bullies are getting any kind of reaction, this keeps it going. Talking to your son about why kids bully helps a bit, too. Telling him that those kids are trying to look tough, but are usually trying to pick on someone else so that they don't get picked on.
If ignoring doesn't help, he can be more direct with them. Simple, direct statements are best. "leave me alone", "don't talk to me like that", "I get it, you don't like my glasses, is that it?" (I tend toward sarcasm sometimes!).
Beyond that, it's time to step it up a little with the school. Check in with the teacher, find out if they have passed along your concerns to the recess monitors, etc.
I am definitely not a 'kids will be kids' kind of mom. Teasing is just as bad as physical bullying and I don't let kids get away with it. The school does have a responsibility to protect your son- from all of it. Having said that, I know that the most attention should be brought to the teacher. She has the most direct contact and therefore the most influence. If you present a spirit of cooperation, they are usually helpful. If they are not, by all means see the principal.
Also, sad but true, this will probably not be the last conflict he has that may require adult intervention. It's such a fine line to go between advocating for your son and becoming the 'problem' parent that teachers and administrators no longer take seriously.
Finally, find some books about kids who wear glasses, standing up for yourself, etc. Fortify him with these resources. And find some guys on tv he likes that have glasses!
Good luck!