You can't say those 18' snow tires are not useful. Width of a snow tire matters more than size of it. As long as they're not too wide, he could run 20' tires safely if he wants to. I believe those are 225 width tires if he runs factory spec, and 225 is still an acceptable width for snow tires, not too wide. I ran a set of 225/45-17 this past winter without any problems.