I have a player that I offered a scholarship at the last minute to fill an opening (I had lots of scholarships). He is a 411 overall and isn't very good on defense or offense. I am thinking about redshirting him to give him an extra year of development. I didn't inform him earlier of a redshirt.

What do you think? Is this a good/bad idea? How bad will it hurt me in my first season?

The question is do you really want this guy for an extra year? If he has High Potential in at least a few core areas then maybe but otherwise I would say get rid of him as soon as you can.

To build on what Weena said, you either want to give a redshirt to (1) your best player so you can really benefit from an extra year of IQ or (2) a good player who might not fully fill his potential otherwise. In some cases, these are the same player.

Basically ask yourself what you gain by having this player stick around for another season. And as Weena notes, if he isn't going to offer you much you probably want him to graduate instead of sticking around.