Ugh, this post actually makes me feel ill. I'm 28 years old now, but throughout my life my mother made awful choices in men who would eventually become my step-father.

Let me save you both the trouble right now: You don't like this kid? Then get the hell out. Taking responsibility over a kid who you clearly don't like will fuck him up much worse than just staying out of their life and letting him figure his shit out. He's a tiny child, behavioural issues like this popping up are NOT uncommon. Unfortunately, so are selfish men who expect a perfection relationship from mom who is already carrying extra baggage. You intruded on THEIR life, if you aren't liking what you see then back the fuck off. The last thing this kid needs is a selfish step-father who they can clearly tell doesn't care about them, or favours the mom/daughter and isolates the son.

It sounds to me like this kid may have some mental health issues that may or may not develop over time. He could get better, he could get worse, it's kinda hard to tell with someone who is 5. But a relationship is built on love and trust, and this lady comes as a package deal. If you don't like the entire package, and can't manage to love and accept every part of it, then it's doomed to fail. It's a dealbreaker, a compatibility issue. Something that is not compromisable.

I don't care if I sound harsh, get your shit together and do what's right for EVERYONE. And learn for the future.