All of the above suggestions are great. Someone mentioned that the "stinky" comment may have simply been observational rather than bullying. When my DD told me someone had said her lunch was gross, I reminded her that she's gotten opportunities to go places, see things, and eat foods than many children haven't been able to, and that's something to be proud of. Maybe OP's DS could offer to share something individually sized out of his lunch with the girl? Or maybe he could invite her to tell him about her favorite foods.
In case it has progressed to bullying, I work hard with my DD (whether I'm successful, who knows) to help her find her voice and not to be a victim. I was bullied as a girl, and I never said a word about it, not to the bully nor to anyone else...and that's the kind of person bullies like to bully. I also think it's a very good idea to work with kids on not bowing to peer pressure long before they get into situations where they're around drugs, sex, etc. I encourage my DD to practice a "Mommy voice": firm, confident, and assertive rather than whiny, angry, or submissive. Might it work to announce to any children giving him a hard time, "It's my lunch, and I like it. Let's talk about ___ instead"?