Thanks for all the nice words and observations. Here are some answers to some of your comments/questions:
He's estimated to be 2 years old. His teeth are nearly spotless - no plaque whatsoever on his back molars, and only a tiny amount on his upper canines at the gum line.
Cattle dog? Wow - interesting! I would have never thought of that, but sure, why not? And yes, I think he is going to be a Velcro dog. He's acting as my armrest as I type this response right now...
Intelligent/human looking eyes? I definitely think so! He's very smart, and he's already picking up on behavior expectations in our house. I caught him marking the corner of our bed the first day home, and I chastised him sternly. He hasn't done it since. I put a belly band on him for good measure - lol. He was just neutered Feb. 18th, so he still has some hormones at play (I was told that it can take up to 60 days before they're gone). I only use the belly band if I'm busy and can't watch him closely.
He has an incredible vertical leap for his size, using his long legs like a springboard. He stood parallel to the couch today, then was on the couch with all four legs at once, almost like he levitated up there!
Terrier? A resounding yes. Birds and squirrels get his attention BIG time! I wouldn't be surprised if he could catch one, if left to his own devices.
His most unique behavior so far? He can completely cover himself up with his blanket so that none of him is visible. I've never had another dog that has done this. Perhaps, he learned this behavior for self-preservation while out on the streets as a stray.

Yes, we believe that he will definitely keep us on our toes!