You don't say how long your partner was divorced or otherwise apart from his son's mother. But if I've got the math right, you were single for a year, then started dating your partner, and within 6 months of that you are living together. (I'm not sure how long it was from first date to moving in.)
So neither your child nor his had more than 6 months to get to know their parent's dating partner before, suddenly, you are a living-together family. You are in love and enjoying all of this, which is great - but neither you nor your partner really took the time to let the kids get adjusted to this.
So, even with teen hormones and moodiness and possible selfishness ("It's all about me"), I can't blame this teen boy for resenting how quickly his father set up a new family. If your partner has custody, your "stepson" only had 6 months of knowing you before his world was turned upside down again (the first time being after the ending of his father's relationship with this mother). If your partner does not have full custody, then his teen only had a few weekends of getting to know you, and BOOM instant stepmother and instant step sibling. He doesn't know where he fits.
So yeah, he's resentful. He is resentful of you. If he doesn't live with you, then he's resentful of his father seeing your child every day and himself only on weekends or every other weekend, whatever the visitation schedule is. And he may be testing you or his father or both. "Let's see how much I can hurt the younger boy, let's see how naughty I can be so my father rejects me which is how I feel anyway, let me punish my father's girlfriend by hurting her child."
I'm a stepmother. I know what blending families is like. When my husband and I got together, his kids didn't meet me for a while. When they did, it was over dinner. My husband (then my boyfriend) shared an apartment with another divorced dad and they had visitation on opposite weekends. So my husband was with his kids alone on one weekend, and with me on the other weekend (when his apartment-mate had his own kids on the premises). My husband and I had to live together financially (other leases were up), but I literally left our new apartment on the weekends he had the kids, and hid my clothes in the storage areas so there was no sign of me there. It was a huge hassle but we did it for the kids. Eventually I spent a night, then two, and so on.
Your partner's son sees you as an interloper. Your distrust of him is going to be evident in every expression and comment and action you take, but you want him to like you instantly or stay away permanently. Neither of those is a good strategy.
Please get family counseling with someone who specializes in these issues. Your first step will be to admit that you two didn't do your homework or lay a proper foundation for this new family. You let your love and passion and impulsivity rule the day. The first thing that counselor will probably ask you to do is admit to this teen that you handled it badly. And remember that he's at the age when he's going to be impulsive about his growing feelings of sexuality and attraction, and you and his father have basically said to him, "Give in to those feelings after just a few months of meeting someone. Profess your love and dive in 100%." And you know that he is getting the opposite message from every sex ed class he's in! Teens do not like seeing separate rules for adults and themselves, so you have to confront that and do a much better job of showing the maturity of your relationship and the sincerity of your parenting.
Blended families don't just get easier. They improve when you work at them constantly. Get professional guidance to help you do this.