We spent a solid few hours at a private dog park (not typically frequented and the best hope for letting him meet our other two dogs on neutral territory). He never had a nervous or slinking posture, but he was more interested in observing at first. He eventually ran and played with the others a bit, but I believe this dog is what you are talking about when you say "old soul". I can hardly believe he's 8 months old. He likes to get out and walk on the leash (which he has done so well with considering he's not spent much time on one), but if I stop, he sits or lays down until I'm ready to move on. It's not formal training, either, it's like he's content to just be with me. He did take a keen interest in the bin of basted rawhide bones at the pet store, but otherwise it was an investigative sniff here and there and then he just walked on.
I had family over last night which he handled well (it might have been a little soon since he hadn't been with us more than 36 hours but he handled it well). He met my roommate's dog for the first time and got a little lippy with her, but it's because she and my dog were growling and rough housing and he acts like a referee around that kind of behavior. We're going to take another week or two to let him adjust and then get him into professional training. I'll work with him on the basics in the meantime to help the bonding process along.
Thanks again for your time!!