It was his blank and lost expression (along with head on the floor) that I noticed one day. So I just helped him out instead and then started getting involved if others were bullied. My general advice was to stand up for yourself, even if I were the bully again and have no pity. I'm currently still friends with most of them and others just acquaintances.
I didn't really said sorry. Well that was already my permanent apology... by helping others get emotionally stronger, which in turns adds confidence and stepping in if needed. I've been against any type of bullying ever since.