I would call in a 'dog whisperer'!! Definitely consult with someone who's methods and goals resonate with yours. Say, if you are vehemently opposed to choker chains, don't hire someone who tells you to use one, keep looking.
I actually watched "The Dog Whisperer" at a friend's place the other day and was really quite impressed by the man's motivations and methods :) I only watched it once, and it's a completely non-violent way to communicate with dogs and to get them knowing their place in the family pack.
My sister adopted a dog that turned out to have some pit bull in it after being told it was a mastiff x lab (no way was it that, it probably had some boxer in it, but def no lab and no mastiff!). They loved the dog but he started to get a bit nippy. She'd been taking him to obedience training for months and months and couldn't get on top of him and does not have the internal resources to put the energy required to get this dog in line. She gave him up and an active, child-less couple has taken him on. In her case it wasn't a case of getting a dog and just deciding he wasn't right - the shelter mislead her (knowingly or otherwise) because he was most definitely not the breed she expected. Though, even if he HAD been a mastiff x lab, she'd have been in for a couple of years of work...she's really not a dog person, she needs a lap dog for her family! In hindsight, it's not the dog that's the problem for her, it's her thinking a dog should fit in and hope like hell it doesn't need stacks of training! Both parties are better off without each other in that case.
Good luck, he sounds redeemable :)

