Too many variables here! Your son is going to have to judge the situation for himself, at the precise moment it happens.
Sometimes the correct response is to ignore the bully. He might go away. But sometimes ignoring the bully can make you look weak and just encourages the bully to try harder to get a reaction out of you.
Sometimes the correct response it to make a cutting comment back at the bully. He might realize you're going to stand up for yourself, and move on to a weaker target. But sometimes this just engages the bully and sends him into a blind rage, and you end up getting hurt.
Sometimes the correct response is to tell a teacher. The voice of authority might be all it takes to get the bully to back off. But sometimes this results in you being labeled a "tattletale" and now the bully and all his friends are out to punish you. And sometimes teachers themselves become bullies, blaming you for making a big deal out of nothing.
Bursting into tears, or in any way dramatically over-reacting, never works, but I'm sure your son already knows this.
Your son has to figure out the right response himself, by watching other interactions and seeing what works and what doesn't, and what the correct response is to this
particular bully. It's not the same answer all the time. It'll vary, even based on who the bully has with him, or where and when he's doing the bullying.
It's not about physical size. Small kids can be bullies. Big kids can be victims. Your son's best defense against becoming a victim is to have a network of reliable friends and stick close to them. Confidence is his best shield. He needs to keep his head up, his shoulders back, and make eye contact. Believe in yourself, and other kids will believe in you.
Good luck to your son! (And please remind him not to avoid the bullied kid - if he can learn to stand up for victims, that will make him even less likely to be a victim himself.)