He is.

Now, stop the move, inform your SO that he cannot change the plan at this late point and that he was sneeky about it makes him and his failure to launch spawn a collective write off.

Don't do this. It never ends. The lying never ends, the manipulation never ends, the spawn are never truly gone.  If you go through with this you are destined to be in a less than position in this relationship and the failed family spawn will always trump... you.  Always trump your relationship.  

Don't do this to yourself.

Tell your SO he is the one moving and his spawn is not welcome. Flip the script. See if he is all in with you, or if he is a spawn worshiping moron.

Take care of you.

you may have enough time to rent yourself a cute little studio or patio apartment. it will give you time to really get to know him better because I'm guessing the man he showed you is not the same man who deliberately hid this from you. you can "date" while he gets his issues dealt with. Starting with why exactly he duped you. 

"My SO now feels torn between me and his son so I now feel bad." Did your SO tell you this? He's full of shit. If he feels in the middle he is the one who put himself there and he's being manipulative and trying to put the onus on you to suck it the hell up and eat shit. Don't fall for it. 

I wouldn't even step foot in that house. Can you have your stuff rerouted back to where you are? You do not have to follow through with the move. I would not. Your SS isn't going to move out any time soon. I had to pull quite a stunt and go buy a house when my SS was 23 and never worked a full time job or went to school since he graduated HS at 17. DH refused to even consider putting him on a launching plan and he'd act like I wanted to run over and change the locks before SS got home. I bought a house and told DH he can either stay with his adult babies forever or come with me. He had 48 hours to make an irreversible decision. The reason for the 48 hour deadline was to put an end to the manipulation and time-buying tactics. I was sick of hearing DH claim that he was waiting for SS to launch naturally. At the rate DH was going, he was creating a dependency and never requiring anything from SS and he was never going to launch naturally. I also wasn't going to tolerate DH changing his mind to buy time or to use me as a booty call. We had been married a long time already and I warned DH to choose carefully because I will not be his booty call. I would have no choice but to move on with my life because I will no longer tolerate an emotionally unabailable man who cannot or will not be all-in with me because of his obsession with his adult kids. I was on the lookout for all of the manipulative tactics. So, trust me, do not even move in. If you do, you're teaching your SO that you'll cave and that'll set you up for nothing but heartache and you'll end up leaving later rather than nipping it in the bud. If you enter that house for a period of time and continue with the relationship you're opening yourself up to a lot of heartache and damage. If you think you're in pain now wait until you're continuously put last every single day for a period of time. It's soul destroying.

If I sound bitter, it's because I am and I'm that way because of the dynamic of what you describe in your post. It's pure hell. 

Why are you feeling so much guilt over this? You were LIED to. There are lots of options here, not just sucking it up and moving in with you SO. In fact that would be the one thing I would NOT do. If he can lie and hide things from you now, it will become easier for him as time goes on.

Your SO changed your living arrangements without even consulting you. Think about that. Sounds like SS is the partner and you are the child.

So, here is what you are up against:

1. A partner that lies by ommission and puts/will put the failed first family ahead of you.

2. You mentioned more than one skid. How many? What ages? It will definitely be a you vs them attitude, and if you try to establish yourself, there will be a great amount of pushback. Your role will not be queen of your domaine it will be dormat, maid, chauffer, cook, bed wamer, bill payer.

3. You have already sold your house and left your job, and are leaving behind a community. Tme to take stock quickly. Firstly find a place separate from SO and his brood. Get that new job. See what is up with things. 

You take care of YOU. Do not feel like the AH.

Don't move friend. We will be here if you do and respect your choice but please do not forget you still have options. This is not good and you need a little more time to see if this guy can prove himself. Right now his actions show he's not worth it. 

With everyone else, it sounds like he's trying to trap you, especially since it doesn't sound as though he is even trying to show you that he is contrite. The only thing I would add is that if you decide to stay with him and get a place of your own, which you should, don't sleep with him or let him sleep over. He will then be able to have the best of both worlds by being able to keep his son in the house and booty call you whenever he wants. 

Where are your friends and family? How far away is this new town? Do you have a job lined up there?

At this point,  I would be deciding, based on the answers to those questions, whether I'd be moving to a rental in your current town or the new town.  Under no circumstances would I be moving in with the SO. 

How old are you?

 I've only just discovered this and I move in 2 weeks. My SO didn't tell me as he knew my feelings on this.

What he's done is all kinds of wrong. There is no way in hell I would move in with him. You are NOT over reacting.

"this isn't about you it's about my son's university choice". 

This is just so disrespectful. I mean, it is about you, also, you gave up everything to move based on certain expectations. He owed it to you to tell you as soon as he knew.

He is going to keep defending his position, using his Dad card, so I'm not sure how far you will get talking to him about this. He will also probably say that he didn't tell you as soon as he knew because he didn't want to lose you or for you to be upset. Neither are good enough reasons for not being honest.

He needs to be shown that what he did was unacceptable. If staying where you are is absolutely not an option, if you have to live with them temporarily, I would try my damnedest to get a place of my own.

I would start looking for full time work, redirect my belongings into storage, find someone looking for a roommate and END this relationship. Then take the time to decide where you want to live - whatever is best for YOU.

You cannot built a life with someone who lets you take all the risk and then lies by ommission on the one thing he KNEW you would not be okay with.

If he'll lie about this, what else will he lie about?

This is absolutely about you. It changes where you live, it changes the dynamic of the home & he purposefully did not tell you. Moving into someone else's home is a lot different then starting over together. When someone tells you who they are, believe them. Stay with your kids, find a room for rent, & rebuild your life where you are. You will have far more peace then with a man who doesn't take your opinion into account. He won't put you first, so put yourself first. 

That he made such a decision without consulting you is a huge red flag. It sounds like you had planned on buying a house in the new city?

I'm so sorry you are in this situation. It IS a major red-flag warning with sirens and is telling you of what is to come unless some miracle occurs. Your SO lies by omission and takes the coward's way out, then ends up getting what he wants. That isn't cute (I know you don't think it is), but it is very immature and very harmful. 

My suggestion to you at this point is to leave most of your things packed when you move (or put them in storage--that might give SO a big fat CLUE about where you are with all of this) and begin making alternate plans for yourself. Again, I'm really sorry. 

PS How long is his son planning on staying? 

I'm sorry this is happening. I agree with everyone else here. You should do what you can to avoid moving in. How he responds will let you know if this is the guy for you. He'll either realize he needs to begin prioritizing his partner over his ADULT children and make moves to center you in his life or he won't and you can choose to move on and find someone ready to live the life you want. 

I think we're all so angry on your behalf because so many of us have lived a similar story. We were promised one thing but then it shifted and somehow we got bamboozled into what our full brain tells us is unacceptable. But our hearts, having been promised the stars and the moon are still hoping, still wanting to believe even though our brains are throwing up a parade of red flags. 

We worry for you because we know what comes next. More promises that shift, more lies of omission, more compromise that expected but only from you. We are so strong in our words because we know how weak you can feel when his words beat you into accepting the unacceptable. 

I can't be the only one who is dying to know what you decided to do and how things are for you now.

Update. I ys been two months.  Hope you found a place by yourself.  This man is on the crazy train with his DS.  NO place for you to get on.  He made his decision on "who's more important ". Answer DS.