I'll add to the others: Have a brief talk with a parent of the bullies. Your son's problem may get immediately better. YouHe might even get an apology ... but I wouldn't count on it.
Now, while it is true the apple usually doesn't fall far from the tree, thisspeaking with the parents will get you much further than "talking to the administration."
Make it clear to the parent that your expectation is for the bullying to cease. A local police officer could mediate on a face-to-face with the parent whose behavior is awful.
My kid has been the smallest since he was out of the womb. There is a disappointing amount of change that a school will make to accommodate the smaller kids. Fighting the administration will mostly just be frustrating.