Anybody would loose confidence if they are bullied, and to be honest you wouldn't smile much or have a sense of humour - your life could be hell if you are bullied at school.
I think the bullying needs to be addressed first. I would want my child to tell me what was happening every day, go to the head teacher, and if you have no joy there I would speak to the governors.
Some schools deal with bullying very well, and some don't. Whether he has Aspergers or not - he should not be bullied. He will learn to live with Aspergers, as we all have to learn to live with ourselves.
I think telling him needs to be handled very sensitively = can you find some books for him to read. Tell him about people who are famous with Aspergers, like Michael Jackson, Bill Gates and Andy Warhol. They say people with Aspergers, usually have a gift, and are really gifted at something, like Art, music, computers etc.
It is very difficult to diagnose anyway - but bullying is bullying and that needs to stop.