Our DS is quite sensitive, he is only 4.5 years, we have always told him to tell someone rather than to react himself. We are teaching his resilience which he's now pretty good at being.
We changed that at school, where a child was hitting him, teachers trying but to no real avail, so he needed to hit back. The Problem was then sorted.
BUT, he hates being "unkind" (hitting, kicking back) he still just tells the teacher if the other kids don't do the right thing, he is basically a snitch
but that's what school tell him to do.
I do worry though as he gets a bit older, he needs to deal with things himself. It's good that he has proved he can, but he just hates doing it.
I think the other parents should look into why your child is always upset, I know I would if one of our DS friends was always sad on our meet ups. What have the other parents said?
Some parents don't like getting involved, it's a lot easier claiming the more sensitive child needs to change and toughen up rather than the rude or unkind child learn some manners. Although in this situation it may not be the others issue, I suppose until you see or know what happens it's hard to guage.
I wouldn't like my DS confidence constantly knocked by his friends, but resilience is also vital in kids, so work on that as well. At 9 I suppose he knows if he wants to see then or not, and yes, try to cement other friendships he is happier in.