[matese]

Happy to hear they are neutered, chipped and blood work turned out good.

I don't think Sam needs obedience training, with his aggression obedience training will not help that. A dog behaviorist will work with him to see what triggers the aggression and work on that. Something in his past has caused fear in him when someone reaches out to him, he's scared and reason he attacks, he's protecting himself, someone in his past hurt him, maybe beat him. He's young enough to have that trained out of him, you need the proper trainer for that.

You mentioned you held him by his collar when he tried to attack him, collars are for tags only, when on a leash they should have harnesses on.

It's horrible what ppl do to these sweet loving dog. Please find the proper trainer for Sam before you get rid of him. If it has to come to that, please follow Lovetodream advise, turn him over to a yorkie rescue, please to not bring him to a kill shelter. It's not his fault, someone made him that way, he's young and he's fixable with the proper training.
Please keep us updated on how Sam is coming along.