It sounds as if his “volunteer” work is what would be called “networking” in any other context; he’s got a business and he’s getting his name out there. While this is not illegal in any way, it seems inappropriate to allow him to do so on school grounds and apparently school time. Unless, of course, other businesses are given access to students and school facilities in equal measure.
But all that aside, if he’s issuing insults along with his apparently very slanted advice, he shouldn’t be at the school at all. College selection/application is a crazy stressful time for HS kids; the last thing they need is an in-house scold.