I'd have a word with his teacher/ councillor and see if they can help. Schools have a responsibility to deal with bullying. They can probably let you in on what the issue really is. It may be worse than he is admitting to as well. In the meantime make sure he has a safe place to to at lunch (library / club/ table?) and that he has some coping skills for bullying that are age appropriate. It probably is something that will pass but you want to make sure he isn't turning himself into an easy target because he doesn't know how to handle it.
Sorry if I sound negative, I think that 12 is a vulnerable age and we can forget as adults how utterly devastating it can feel to not have friends and feel powerless to help ourselves. As gt and with anxiety your DS just might not have the ability to suck it up and find new friends without support.
