Is it just this year's teacher? In which case I'd log all incidents and put in a formal complaint as Bunny says. If it's been a problem over the years then is it something that he's not understanding? If so does your school offer learning support or something similar.
Another way to perhaps increase his confidence is to do things outside school that he's good at - maybe sports, or cubs or something and keep praising all the time.
I know what you mean though - no disrespect to teachers at all, they do a fantastic job in the main, but I wish some of them would just think before they spoke.