It's understandable that he's angry that her new husband did that to his child
Angry? No doubt he's livid. Can't blame him one bit.
What DAD needs to do is call the police and CPS, as well as seek an emergency custody order.
Where is stepdad now? If Mom didn't throw him out for punching HER child in the face (and just newly wed....I predict a very bad marriage and mom may receive a few punches herself. A man who has the gall to do so...right away?! Wow), it's likely she might never see her child again.
CPS often insists the abuser (esp if he is NOT related to the child) leave before allowing the child back.
So where does mom stand? Weakly defending and sticking by the creepy moron? Or her son? If it's not her son, she's likely to lose her child altogether. Any mother worth her salt would throw the bum out immediately...not whine to bio-dad she wants to see son.