not sure what I would say myself as it's very hard to bond with someone else's kids and of they are very different from you then it can be even tougher cause there may be no common ground.
He is also the opposite to ur style of parenting which makes it more difficult and probably when you see some of the way the child is parented, you may feel a little disgusted by it- I know this happens to me alright
You could do honest - at least everyone knows where they stand then
But if you feel magnanimous try a gentle approach that includes parental responsibility on your partner side while attempting to portray you as sensitive and thoughtful
Perhaps you could say something like
I do sometimes find it hard to know him. He seems to do a lot of Lego and that makes him happy but it's such a solo activity that I don't want to crowd him. Maybe we could get him into XXX sport like your other children and we could all do that together? What do you think?
Or
I think we have had some teething problems settling in alright, I have noticed at times he seems to have some real problems with my girls and that upsets everyone. How can we work on settling these teething problems? What do you think we can do to help on both sides ?

I tried this sort of gentle approach at first when I was first starting my relationship in an effort to keep the peace.

Now however I am past that cause it didn't work for me. now I deal in strict honesty and disengagement
Sounds like you raised some good eggs yourself tho- good job you!

This is not going to be what you want to hear, so I apologize:

If you've just moved in together, and this is how you feel already, you should consider moving out.This kid is never going to change. This is his genetic makeup, you can't change it.

Your partner is not going to change. You can't change the dynamic between your partner and her son, it is what it is. The only thing you can change is you. So that leaves you two options, either engage more or disengage. But if you don't like this kid, you never will.

I was where you are 5 years ago, it does not get better, only worse. You are never going to click with this kid.

In my case, I have a lot of guilt because the child I'm dealing with has severe issues (still soils his pants at 10, has severe ADHD and some kind of developmental delay). Although my other SKID, my DH and my own bio kids make up for it... this kid has made my life miserable. If we didn't have a bio kid together I would have moved out. I'm surprised I've made it this far.

Meanwhile, my hardresser chose a different route. Her and her boyfriend have decided not to move in together until their kids are 18. Their youngest was 9 when they started this. They go out as a couple, go out as a ffamily, spend time together, have sleepovers, etc. They don't live together, and they're happy!

I'm envious.

Welcome Heathrsant, I have been using this site for my sanity for many years now, to receive and eventually to give advice. My picture has always said "new" but I am not new anymore. Anyway, I try to give the most honest advice I can. Your post struck me in its similarity to my early years as SM, although I have no biokids. My SS was about the same age as yours when I moved in with my now husband. SS was (and still is) lazy beyond lazy. He had no friends and did nothing but loiter within one foot of my husband's body at all times. He never had any responsibility, no chores and was handed everything he ever wanted on a silver platter. He never interacted with me. In the beginning I really tried hard with SS. I tried playing games, projects, TV shows, cooking, holiday stuff etc. The extent of his conversational skill with me for the past 7 years has been "uh huh" "yep" or the shoulder shrug. The child would walk into the room where my husband and I were having a conversation, step between us with back to me, and start chatting with his father as if I was a ghost. When I brought up these issues to my husband, he said that it was MY fault for not interacting with SS. One day (one of the last straws) I gave SS a hug as I often tried to do back then (makes me cringe now) and my husband actually told me I was hugging him wrong, that it wasn't affectionate enough. Over the years the child's behavior has been a constant argument between my husband and myself and he has never taken my side. He defends this child no matter how blatantly disrespectful he is. Because SS was never disciplined and counseled to improve his interactions, he is socially no better than a 9 year old in a 17 year old body now. After so many years, it has driven my husband and me apart and made me truly detest this boy. The only thing that gets me thru the days is counting the weeks until he goes off to college. Unfortunately, I have to agree with LikeMinded's post. You have just moved in with this woman. It is already clear what her son is like and more importantly it is clear that her loyalty is not to you. As LikeMinded said, this will only get worse. It is not your fault for disliking and unlikable child. As bad as it seems to leave after only a few months, it is better than after 2 years or more. In your case, you have your children's well-being to consider, and it sounds like they are well-rounded, pleasant children who have a great future. Household stress because of this boy and his enabling mother will eventually erode the happiness of your children. I would guess that your partner soon will start resenting you and your kids because your kids are so great, and hers isn't. It will likely just get uglier as she tries to blame everyone except her son for the household turmoil. I stayed, hated every minute of the 50/50 custody, and hope my persistence eventually pays off then SS is gone. Would I do it again if I knew then what I know now? Absolutely not. Blended families are so hard to begin with, and if your partner isn't on your side, hard becomes impossible.