Best solution (this actually works more often than you'd think): Give him the 15 mpg this season. Then redshirt him NEXT SEASON. Typically that will work, unless he's a JUCO. Of course, it doesn't always work and it's far from foolproof, but I have had it work twice in the past at DIII. I haven't been in DIII in forever so I don't know if it's less likely to occur since some of the past updates, but I think it's still possible.
If you're planning on redshirting a kid though, never promise him minutes or a start. It's just not worth redshirting him, because his WE typically takes a huge hit. I'm not sure if he could transfer as a result, but it seems like a logical result. I mean, in real life if you sign a kid and promise him minutes, then choose to redshirt him, he's obviously not going to be too happy with you. But the rep hit would only occur if the player transferred, not simply due to a WE drop.