OP here: the update isn't good. We borrowed a live trap from the SPCA. So far, he hasn't been caught. He's reverted to feral, but since he has been an indoor-only cat for the past years, we think the humane thing to do is to trap him and return him to the SPCA. I am well aware of what will happen to him there.
On the advice of 3 doctors, I have started rabies shots. I have been worried sick - the chance is very slim that Tigger could have been in contact with a rabid animal, but none of these doctors were willing to tell me that it's not a concern. I am almost done with the course of antibiotics that I started last week, which is pretty much routine for bites like these.
I have mourned for the loss of Tigger, but he has never been a comfortable pet. He has spent every day for the past 5 years under a bed or behind the couch. He would come around in the evening with his brother. I could never pick him up. To say that there is something wrong with him would be correct.