Newly adopted dog has bit three times now, what to do?

Adopted a 9 month old mutt a ~week ago and he has already bit me twice and my husband once badly enough to need stitches. He has an obvious trigger: being touched on his back half (vet checked, no medical issue). But its not every time, I can pet him all over or even pick him up and it's no problem, until it suddenly is. No warning growls, sometimes will air snap first. I've never dealt with a dog so quick to bite before and am seriously considering returning him to the rescue.

I've searched the sub and seen mentions of hiring a behaviorist or trainer, but want to know how how quickly they would be effective. I'm not comfortable having a dog where its russian roulette if he's going to bite me this time I put on his harness or not. I have muzzle trained him now, but that's only a short term solution, I'm not sure where to go from here? Am I overreacting and this is still part of the 'getting settled' phase?