Beck is Sam's father figure in a couple different ways. First and foremost, he is responsible for making Sam a werewolf, for better or for worse. He says he did it to rescue Sam from his flighty parents, but we think Beck was just lonely. Faced with a wife dying of breast cancer, and his inevitable fate of being a wolf forever, Beck just wanted to pass his traits on to a child in one way or another. Making puppies is one way to do it.
He has to pass his traits on because the wolves need someone to take care of them. An insider, if you will. But after seeing what he put Sam through, he only finds volunteers for werewolves instead of transforming people against their will. Although we have to wonder how in detail he gets with these people. Do they realize they'll look like “a placenta covering a terrifying, feral infant” (50.7) when they change? Because thanks but not thanks would be our response to that offer.
Anyway, whether or not Sam would have had a better home life as a human child is up for debate. Even after realizing that it's Beck's fault he's a werewolf, Sam knows that “ever since Beck had taken me under his wing, I'd been surrounded by family” (27.11). Beck is family, and Sam would do anything for him. That might just be the measure of success for a father.