It can be difficult to blend families.  There can be different dynamics, parenting styles and other factors that feed into it.

LIke you have two kids and he has one.  That is an imbalance right from the start.  Then his son has a bossy personality and it turns your kids off.

I guess it might be worth exploring if you could do at least some visitation on off schedules.. the kids not being in the house at the same time?  Otherwise, both of the parents need to help their kids be more tolerant of the other children.. like your DH stepping in if his kid is being too overbearing.. and you stepping in if your kids are being exclusionary.

Agree with ESMOD.  It sounds like you both feel like your kid(s) is/are fine, and the other's child is being difficult.  But neither of you is willing to do the parenting required to make the kids capable of interacting.  In a job you don't just get to say I don't like to work with that person and close your office door!  What kind of life skills are you teaching?

That doesn't mean they have to be best friends.  But his son needs to be less bossy (it's a childhood trait that needs parenting!), and your kids need to learn to be more inclusive.  They don't have to interact all weekend, but neither should they be allowed extended periods of bossing nor excluding.  And no child should be able to "choose" not to come see their parent, so there should be no threatening of not coming to visit you. 

Sounds like the adults need to agree on some family activities (with everyone involved to model behaviour), and also take the time to parent their children to a more mature level of interaction, rather than just throwing your hands up and saying "kids will be kids" while silently meaning "your kids is a brat". 

So his son is just the way he is and your kids are the ones who need to change? 

Your SS needs to learn the lesson that if you're a jerk then no one wants to play with you. I don't think you should step in at all, this is something kids can work out amongst themselves.

If your SO forces you to make your daughters play with him even though he's being a bossy jerk then all that's doing is teaching SS that he's entitled to get attention no matter how he acts. It's even more problematic since your daughters are girls. This is how you raise a man that thinks he's entitled to women doing and giving him whatever he wants. Scary. 

"Well, if there is nothing that will change your son then there is nothing that can change my daughters. I find that to be a very silly statement when we're talking about kids who are still learning to socialize. Instead of expecting my daughters to accept your son as he is, perhaps we should work with both sets of kids to try to get along better."

Your DH is being unreasonable if he thinks your kids should just accept his son's bossy behavior in order to keep the peace. It's also a lazy parenting mindset to just "accept" socially unacceptable behaviors and not work to correct them.

I don't think you need to divorce PROVIDED that your husband is willing to work with you on blending the kids (or accepting that they won't blend and will lead different lives under the same roof, which is also acceptable so long as everyone can be civil and respectful). Family counseling might be a good way to go if you all can afford it. There are many self-help books, blogs, and websites about how to blend. Your DH also needs to learn how to temper the bossiness in his son. Assertiveness and leadership are great qualities, but they can become tyrannical very easily. Your DH can teach his son how to best present what he wants, set boundaries around those wants, and also how to compromise so everyone gets something they want. He needs to nip the bossiness when he hears and sees, correct the offending behavior, and redirect SS elsewhere. SS will learn between DH correcting him and your daughters delivering the natural consequence of not playing with him to adjust his behavior (or at least he should).

If, however, your DH isn't willing to teach his son or bend, then you need to think about whether your daughters are presenting you with a fair argument. Should their happiness come first? Yes and no. If there are adjustments to the custody arrangement or even just keeping the kids separated under the same roof and that fixes the problem, then your DDs aren't being fair by saying they won't come over. That isn't a choice they have, and not one they should be allowed to exercise just because they don't like a situation.

However, if SS won't leave them alone and terrorizes/hits/screams at them, your DH won't stop SS from terrorizing them, you can't switch up the custody schedule to have the kids on opposite weeks, and your DDs are having real physical or emotional symptoms from being around SS (e.g. panic attacks, throwing up), then you need to find a more permanent separation between the kids. "Living apart together" might be something worth trying. Personally, though, if my DH allowed his kids to terrorize my kids to the point that I had to move out for their own health, and all my DH could muster in defense is "that's just how my kid is", I'm not certain I'd respect my husband enough to stay married to him even if we lived apart.

ETA: You also need to take a step back and see how your kids contribute to the problem. Your kids have grown up together and gotten used to each other. Are they excluding your SS just because they can, and does that push SS to be bossy as a defense? Are they letting him play, but only as a side character to whatever they're doing? Do they let him choose activities or do they rule what happens because "majority rules" so he never gets a chance to engage with them on something he likes? Even if you haven't seen this with your own eyes, has SS or your DH expressed that this is happening?

If any of this is happening, you have just as much responsibility to nip it as your DH does the bossiness. And you'll have to do the same work I mentioned above that your DH needs to do. Don't assume that just because your DDs play well together that they are well socialized.

Try a few weeks of "family Saturdays" to observe (and curb) behaviour and to try and encourage communal activities. For instance one week you could make pizzas together and play a few hands of Uno, another maybe a trip to the movies and then a burger, or whatever activities you enjoy. Participation is not optional. It's all part of the learning process. Just don't make it last for more than a few hours - it's not supposed to be an all-day experience unless you're going to a museum/fair/match in another town. 

Are your kids and his kids there the same amount of time? It's a strange thing, but i have my daughter 50/50, and SO has one of his kids 100% of the time and has both on all of his off days. Meaning we get no days without one or sometimes (gasp) all 4 of his kids. There's an imbalance. I have 2, he has 4. I have one ex, he has 2 BMs. We have lots of time without my kids but none without his. I'm often the "odd man out", alone with just him and his kids, but i can't remember a time where it was ever just him, me, and mine. And his are all (well, 3/4) attention seeking with behavior and emotional problems. It may be petty, but it does cause resentment over time. For me, if we just had any alone time, or maybe even (petty i know but this sh!t adds up) some time with just us and my kid(s), i would probably feel better.

As for the behavior issues - my daughter can be bossy. I try to observe her behavior with other kids when she doesn't think i'm listening, and i will talk to her about what i see. She and SS18 were having issues and i had to tell her "it's you, not him.", and suggest ways she could correct herself. I think you should both do that with an open mind, and if you can, it may help things a lot. No person is perfect so there will probably be things both sides can do differently. 

Ask your x to take the girls when his boy is there? 

This is hard and I feel like some of his son's behavior is related to being an only child and then blending. He is seeking extra attention from his dad.

Maybe you can see about a change in custody arrangements. Maybe every other weekend and every Wed. night would be a better schedule. Wed. night would be simple enough with dinner and school bedtimes. Every other weekend would create some time apart from all the kids having down time together. Maybe if you do go to every other weekend, one Saturday can be a bio parent day, the other a family day? The amount of days your DH would see his child would be the same for a month, just the spacing is a little different. 

"That's just the way I am and you can't change that" was a favorite of my ex. I was expected to change to fit "who he was" and not honor "who I was." It's a stupid position to take.

The kids don't have to like each other, but basic politeness and manners is needed just to learn to function in society without being an @sshole.