Follow-up to this post from last night:
Still haven't heard anything from the dog's owner, but after having him for nearly 24 hours, I have a lot more reason to suspect he was abandoned.
He's completely untrained—not only not housetrained (thus far, he doesn't poop outside—he poops on my bed), but also no sense of boundaries at all. He jumps on me and clings on to my arm with his front paws and just will not let me push him away. He also seems to have panic attacks, or whatever the dog version of that would be—he jumps in my lap, stands on his hind legs and tries to get his face as close to mine as possible, and then he just clings on and cries and shakes, and it takes a few minutes for me to calm him down and comfort him. He's a high-maintenance little guy, in other words, and one that's going to need a lot of training and care.
I'm going to take him to my vet tomorrow to see if he has a microchip—if he does, hopefully that will provide either a work phone number or an address. If he doesn't, I'm not sure yet what I'm going to. I'm already half in love with him, for all his issues, so I'm leaning toward keeping him, but he may need more care and supervision than I can provide—I'm not sure he'll be okay staying home all day with Buster while I'm at work, for instance. If I can't keep him, I'll find a no-kill rescue. He's ridiculously cute, so I'm sure someone will want to give him a good home.
Here's a picture of him right now, curled up happily next to be on the couch (I've been calling him Charlie, but as lirael brilliantly suggested, if he's making Buster jealous, I should really call him Annyong.)