It's not just the 7 errors. He's made many more mistakes than 7. Some still got the guy out- others did not.
He'd probably make some errors at 1B, too, but I still think that's where he'll eventually end up.
Yes, he has some quick reflexes, and that's a big plus at 3B, but I just don't think it's a good idea to put a young kid into a learning curve situation during a heated championship year or two. It's a lot of pressure. If he keeps contributing to losses due to his glove/arm, his great offensive abilities will be partially diminished.