I voted to give him another chance, but I'd do so on the condition that he apologizes and that he's honest with you in the future when he's made a mistake. The most disappointing thing is that he couldn't tell you about the schedule conflict up front, apologize, try to find a sub, or whatever.
A 4 piece band I was in back-in-the-day had our lead guitarist bail on a fairly big (for us) booking with similar short notice. He had forgotten that his wife was graduating from professional school that Saturday, and he wrote "I have chosen to remain married, so I must miss our gig" in an apologetic email. We scrambled, found a sub, and stumbled through. No one was happy about it, but I appreciated his honesty and also had goosebumps of the there-but-by-the-grace-of-Google-Calendar-go-I variety. Mistakes happen, life happens, but the adult thing to do is to own it when you screw up, apologize, and try not to do that again.
Maybe this will be an opportunity to build a better working relationship with your drummer... and/or find a solid backup drummer ... or find someone new who is more reliable. Good luck!