It doesn't sound like a good dynamic to me. The majority of cases of infanticide etc are committed by step-parents... Evolutionary psychologists have a field day...

It is common for father / sons to 'lock horns' when the sons are in their adolescence. It is a part of a boy growing up...

Someone elses son... Is a different dynamic indeed. Different dynamic entirely.

I'd be surprised if this came as a sudden shock, from out of the blue. I'd be surprised if the stepdad was supportive and nurturing before then suddenly turned on him...

The way you posted the thread title it seems clear that you feel you have to choose between them.

What kind of a mother chooses a possessive guy over her own son?

?

It isn't like your son is a delinquent and the step-father is trying to lay appropriate boundaries out of love for your son.

Is it?

Does he really treat you any better?