Hey Scoot,
I was a little fat kid too. I had some bad times when I was young. Always bad at sports, always chunky, and I hated it. I think you should just worry about his physical protection, and make sure that the principal and teachers (as well as those damn hired 'monitors') know what's going on.
He'll outgrow his problems with confidence, but it won't be easy. It never is. Changing attitudes is one thing, but appearance is another. In high school things will change, but getting him into activities that involve him with kids outside of his School's social circle would be best. Someone suggested martial arts....good idea. Make him confident in his mind, and accept his appearance...trying to make him change it will just accentuate something that is not important.
Trust me, I know what I'm talking about. I was the little smart, fat kid who nobody liked. I was bad at sports AND a JW. Now, I'm married to a beautiful woman, successful in my job, and balanced in my life, but I'm still kind of chunky, and it's not that big of a deal. I give my mom a lot of credit for that because she never mentioned my weight, just now and then she would say that one helping was enough, and things like that.
Your kid will be fine, I'm sure.
Ashi