Even if your drummer has three toms and he mics every drum, you shouldn't need more than seven mics for him, and that's a stretch. Kick, snare, toms x 3, two overheads. I often mic our drummer with three mics: kick, snare, overhead. My guess he's miking top and bottom of the snare and every cymbal. If you're running the sound and are just starting out, there is no way you will be able to get a meaningful mix out of the drums. There's no way he will, either. Twelve drum mics is just ridiculous overkill in your situation.
If you are going to give him 12 channels, you need a 24-channel mixer, but telling him "no" is the best option - seven would be generous, and with that you might get by with a 16-channel board. If bringing him to his senses is impossible, I'm with the "have him get his own submixer" crowd. Failing that, I'd fall back on the standard TB solution to problems like this. Fire him.