[matese]

Not knowing much about your two boys makes it hard for us to offer up advise / suggestions. Depending on how long you have had these pups, sometimes it takes a dog a long time to build trust and bond with you. With rescues everything must be done in baby steps. In rescues changes don't happen over night, I have a rescue, he was not an abused boy, he was loved much and spoiled but he came with baggage. I am sure you took these boys for a wellness check with your vet. Did you speak to your vet about Sam's aggressive behavior to rule out any medical issues. My now passed girl had the sweetest disposition, loved all ppl and all animals. When I pick up my babies I put one hand under their butt, my other hand goes under their front legs, I have always scooped up all my babies in that manner. One day a very good friend of mine stopped by, my Matese grew up with this friend in her life and loved them. They picked her up with hands around her middle, she yelped and bit the friend, she never, ever, bit anyone, I told my friend that is not a way to pick her up. I thought nothing of it tho. 2 weeks later I had to bring her to the vet for something, I forget what it was but nothing serious. My regular vet was not there, we saw another vet that was excellent with dogs, well, he picks her up the same way my friend did, around the middle of her body, she yelps and bites the vet. Immediately the vet says one word "Pancreatitis", WHAT he said he was going to run some tests, he believes she has pancreatitis from her reaction of being picked up. Well, the tests showed that was what she had.
I tell you this story because you say you cannot pick Sam up, when you did he bit you, the first thing that goes through my mind is when my Matese was picked up she bit. It's just a thought you may want to pass by your vet to make sure he does not have any health issues that is causing him to bite.
Can you contact the lady you got him from to ask her if Sam did this to her. This would let you know if he has always been a biter. The more you can find out about Sam will benefit him and you.