childless step moms!!!!!!

I'm 24, I've been with my fiancé, Jason, for a year and a half. We've had the best connection possible and i'm not stupid. I truly do love him and can see a future with him but he has a 5 year old son. When we first got together I wasn't around his son much but had those days of dates planned and having to cancel because the "ex/mother of the child" know we had things planned and would pawn off the kid. Once I was around him more I notice more and more that he had problems. Hyperactive, didn't listen, hit people, bit people, throw things, temper tantrums from hell. Over the course of Jason and I being together I could see that I really wanted to be with him but his kid was not what I thought he was. I almost couldn't bare to be out in public with him because he was horrible. All his authorities figures let him do what he wanted and would buy him whatever so they didn't have to deal with his behavior. He was taken to a doctor and diagnosed with ODD, ADHD severe, and undiagnosed bipolar (doctor said he was too young but because the mother is diagnosed we fear he may be soon). I was raised in a family where you don't talk to adults the way he does. We got spanked if we did anything wrong or things were taken away. My mom tells me i'm too strict. But Jason is way too soft on him, almost defending him instead of seeing what's really going on. So my stepson has been out on medication. Before we did we tried oils which helped but we would forget to put oil on him and he would have the worst outbursts and attitude. As of today his behavior has changed but he's becoming more and more sneaky with things. He's been taking things out of bedrooms, taking food but then lying about it. I always thought this stage would come when he was a bit older. But right now i'm just emotionally and physically drained to where being at work days we have him doesn't bother me. I'm having a very hard time trying to love him like my own but I know his mother personally and he's picking up all her habits. I don't want my relationship with Jason to end. As bad as this sounds, without the kid, Jason is my dream guy. But I can't have everything without an obstacle. I just want personal experiences to help me. I want to be able to try for a child later this year but I fear that with the way my stepson is, he would try to hurt any children Jason and I have together. I know right away I'll have people tell me that maybe this relationship isn't the best for me and you're right. The relationship came with this and I knew it already. I just don't think I prepared myself the best. Children are insanely hard if you don't have a child of your own it seems. I'm looking for something to help me. There are days that I love the kid, it just seems like he takes out all his frustration out on us. Which isn't unheard of since he's seen his mother do that same thing to Jason when they were together. Please, stepmoms who didn't have kids prior to your relationship, I would really like some help.