The thing about it taking seven years is interesting, as that's probably how long it was before I thawed out and started loving my step-dad, who loved me the whole time I was telling him he wasn't my dad and couldn't tell me what to do. I'd have rather cut my arm off that use it to do what he told me to do. My sister was always threatening to go live with my dad, and I was always threatening to go live with my grandma.
I also have to say that I tell my kids I'm gonna beat them all the time. I haven't spanked any of them in years, and never spanked them nearly as hard as I was spanked, and I don't think I was ever beaten. My kids' actual favorite Mom threat is once when I was really, really mad at my daughter and yelled for her to come here right now, and she yelled back that she was changing, and I yelled "I'll change you into dead if you don't get down here!" So now, when they want to say that Mom was mad, they say "I'll change you into dead!" and we all laugh until we cry. Some people naturally yell and threaten but never follow through. If he raised another child and didn't beat that child, he won't beat your son, either. But if you raised your son in a "we don't say angry things in this house" kind of way, he's not going to be used to someone who gets what he feels right out in the open and yells. So that'll be an adjustment.
What I see as your biggest problem right now is that you're telling your husband that he isn't your son's dad and can't discipline him. Your husband is now a partner with you in running your household. As an adult, he deserves respect and obedience from any child in the household. As a de facto parent, he deserves the right to have a say in how your son behaves in the house he shares with you. As your husband, he has a right to expect you to come to his aid and create a united front for the children, but instead, you are taking your son's side against your husband, and that's just wrong on all kinds of levels unless he is actually abusive. Yelling that he's gonna beat him if he doesn't clean his room is not abusive.
Do not be one of those parents who think their perfect honor student child would never be in the wrong. I was a perfect honor student child, too, and I'm here to tell you that I used my intelligence to find ways to get under my step-dad's skin whenever possible and then tell my mom how mean he was to me. My sister, another perfect honor student, was way worse than me. Honor students just find subtle ways to get their digs in. My sister and I did everything in our power to make our mom feel guilty about getting married (I was 14, she was 12.) We moved when she remarried, and I had such a big fit about it that they both quit their good jobs and he moved back to where we'd lived before, even though he'd been living close to his own daughters before and moving meant being 5 hours away from them. We were mean and hateful and I actually got in big trouble for hitting my step-sister with a belt while I was babysitting (she was a nasty little creep and she deserved it, and I tell her that now every now and then to remind her what a pest she was.)
Settling into a new family unit is hard, but it's much harder if you don't sit down, plan how to handle situations that come up, and stick together. Your children want to split you up. It's natural for children to want that. Whether or not they succeed depends mainly on you. If your husband is a wonderful man, he will not hurt your son except the hurt caused by displacing your son as the center of your attention. Your daughter is 18 and frankly, you really don't need her living with you if she is able to be on her own. Our job is to grow them up and shove them out, then circle around to make sure they fly. Both of them are using the "never want to see him on holidays" threat to control you, and you're falling for it.
I'm so glad my mom didn't let us break up her marriage. They have been married for 33 years, and I love my step-dad and my step sisters with all my heart. He loves me as if I were his own, but he waited a long time for his love to be returned. Give your wonderful man a chance to win your kids over, and be his backbone while he's trying to do it.
Of course, if he actually did beat his first wife and her child, all bets are off. In that case, he is not a wonderful man and you goofed up.