If there's not a lot of good, I was assuming perhaps it was one part of his contribution, but if there isn't any good at all then how about "Thanks for contributing but in order to be an effective member of the team you're really going to need to work on X, Y, and Z. Unfortunately we cannot commit your patch to the code repository until this is the case. If you do work on X, Y, and Z though you should be at a point where you can contribute to quite a wide variety of areas within this project and will be a quite a valued member! We need people like you with dedication. And don't worry about it, if you look at version 0.a.b you'll see I made a bunch of the same mistakes!"
If you have the time to dole out harsh criticism of someones patch you can't take the time to provide a helpful comment instead of an overtly negative one? "Oh I spent all this time reviewing your code and it sucks." is better to you than taking the what, ten or fifteen minutes to compose a quality reply? I understand the top tier linux manager's time is valuable. But if you can tell the code is wrong, then give reasons for why it is wrong, tell them you can not commit until those reasons are corrected, and do so in a polite manner. In addition, by providing resources, you make your own job easier as the quality of patches continues to improve in quality, you retain talent, and foster new talent within the software community.
If you don't want to provide resources then perhaps a "Hey, if you have any questions trying to work through X, Y, and Z try to contact me here although you'll get far quicker responses on irc.freenode.net #linux".