Wow thanks so much to everyone who replied. I've been reading up on the Parelli methods and am really excited to start some groundwork with him. Personally I feel it might be a combination of saddle placement (thanks so much for the article link!!) and Sarge just missing his old owners.
The environment here is a bit different than what he is used to. We have a large open pasture for them while his old home had multiple, smaller enclosures and he was taken out and ridden frequently. We were told that he had bonded with one of their other horses, Lilly, and also seemed very attatched to their family. I feel that maybe Sarge just doesn't trust us at this point and is confused by his new surroundings.
As for health concerns, Sarge just had all his vaccinations, shots, and a new check-up just before we got him and we were given his records so I don't think that's the issue. Though the bridle we were using didn't seem to fit him comfortably and was not what he was used to either. We've gotten a replacement bridle for that and again, I feel it's probably just a combination of newbie mistakes and him missing his previous owners.
Our plan at this point is to build a round training fence to lay groundwork with him (and most likely Storm too, though he doesn't seem to have the attatchment issues that Sarge does). We've hired some folks to come and clear out about 2 more acres in the back of our property next week so hopefully within a week or two we'll have a training area where I can work on building trust with him.
Again, thanks so much for all of your advice. I'm tryin to take in and absorb everything now and your feedback is very much appreciated!