I'm not a dad, but I taught middle school for 15 years. Pantsing happens a lot, middle schoolers can be harsh, and it's a great time for boys and girls to get a thicker skin.
If you're worried about a pattern of teasing, I'd talk to a guidance counselor. Explain your concerns and ask for her observations about your son. Sometimes, students engage in specific behaviors that lead to them being teased. It's not right, but it's something you can gently work on with him. High school is a lot better.
OK, so I get it. The girl who talks to the window, or the boy who comes to school with food all over his face are kind of setting themselves up for teasing from their peers. Middle school kids are not nice. I hear you. Parents can help their kids learn social graces and make sure their kid takes a shower, combs their hair, and wear pants that are long enough and match the shirt.
HOWEVER!!!!!
^^^Your's is the same reaction from teachers and staff that I got when my son, who was a brand new student at the beginning of 6th grade (we moved in April of his 5th grade year), was being bullied and excluded just for being "new" to the school full of kids who had been together since preschool. Wow....what a great way to put all of the blame on a child and excuse the bullies. My son was a good student, friendly, happy, outgoing, tall and athletic, cute with nothing abnormal or out of the ordinary about his looks/hair/clothing/etc, he just happened to be put in a class with the "Star Athelete" of the grade who took one look at my son and decided that he was a threat and did everything he could to put DS down and exclude him from not only Mr. Star's group of friends, but the girls and everyone else who might possibly try to be his friend.
The teachers excused the behaviors by explaining that the bully's parents were getting divorced and that my son was the "new kid" and couldn't possibly expect to make friends right away. This kid made my son's life absolutely miserable for the rest of 5th grade and most of 6th grade and nobody wanted to help, least of all the teachers who turned a blind eye.
Thicker skin? Yes, my son certainly grew a thicker skin that year....at the expense of a lot of tears and loss of confidence and grades and lack of joining in at school. Thank god he had parents, a football coach, and family that never gave up on him and went to bat for him every time he was bullied so he knew that *someone* was in his corner. Thank god he was tall and athletic and a trained linebacker so that those jerky kids couldn't take it to the next level and physically hurt him. He is now almost 15 and not a time goes by where he sees someone being left out or bullied and doesn't step in. MY SON grew the thicker skin and the confidence and is now able to help other kids who might not be able to stand up for themselves while the bullies got to pat themselves on the back and congratulate themselves for being the kings of their universe. So, I guess MY SON was the lucky one after all. But man, that was a tough year that didn't have to be so bad if we had just gotten a little bit of support from the educators who were supposed to be there for the children.
OP - talk to the school/coach and go to bat for your son. Let him know that you are on his side and do whatever you can to hold that bully accountable. Don't count on teachers to help - I know there are amazing middle school teachers out there, but there are too many more who are just sick of the whole social dynamic of middle school kids and don't want to get involved. Don't let people tell you that it's not a big deal...even though your son will come across jerks his whole life and needs to learn how to deal with them, at this age, he needs to know that you are there in the background supporting him and backing him up.
I would also recommend, independent of the school and bullies, to keep your son involved in whatever he excels at and make sure that you encourage him to brings friends home and hang out with kids....boys, especially, don't need a gazillion friends, just a couple of really good ones.
