As long as the straps are nice and tight, there is no real danger with him unbuckling the chest clip. It's only purpose is to act as a pre-crash positioner to keep the straps over the shoulder.
I'd rather the chest clip just stay clipped... i've been googling all sorts of keywords for this and can't find any product. I did find something on mypreciouschild. com that clips the straps together, I guess for added protection? But it doesn't cover the actual clip itself.
Do you think I might find something in the Child Safety section at home depot or somewhere like that?