Then you not in a partnership. I would re-think marrying this person. You will end up very resentful.

My husband did this to me once near the begining of our relationship. SD showed up at a time that was not the scheduled custody time. He had agreed with BM to take SD for that evening/night. I was not asked about this so I picked up my coat and bag and went out for a few hours without saying a word to him. When I got back I gave my husband the cold shoulder all evening. I did not do anything for SD or play with her like I would normally have. He learnt that night that if he is going to do something that involves me or imposes on me that I must be consulted beforehand and agree to it. 

Now that is not to say that SD didn't come over when it wasn't her usual time, just that my husband asked me before hand if I was ok with it. There were also times when she came over, or even came to live with us that were genuine emergencies and I was ok with that too. There is a difference though between BM is homeless and SD needs to come and live with us, and BM wants a night out with her friends and wants us to babysit. 

Have you married yet?  If not, put on the brakes until your SO learns that he needs to involve you in decisions that are going to affect you.  BethAnne's approach works if simply telling your SO it's not acceptable doesn't.  And if that doesn't work, rethink the relationship, because you'll have a lot of years of being unhappy with the lack of control you'll have in your own home.

If you're marrying someone you should be able to talk to them, especially about an issue like this. These decisions affect you so you should have a say. If the decision affects YOUR family you should also have a say. Being a stepparent doesn't mean you are voiceless. True, you might not always agree with your partner and he may decide something contrary to your wishes. sometimes we just can't get on same page but there should still be acknowledgment of each others feelings and some kind of compromise or expression appreciation for any sacrifice made.  He should talk to you and consider your input. That's treating you with respect. Discussing problems calmly and working together  Respect is key in relationships. 

You absolutely should not be the last to know.  I agree with others, put the brakes on marrying this man until this issue has been addressed, discussed and agreed upon. 

Start with canceling the wedding.  Don't tell him. Just send out cancelations to everyone on the guest list.

When he freaks out, hand him a list of the decisions he has made without you and inform him that crap ends now or there will be no reschedule of the wedding.

If you do not come down on this hard now, it wont end and the misery you are feeling now will just escalate.

Good luck.

Take care of you.

One unfortunate part of steplife is that occasionally things will fall into place in a way that isn't convenient or in a way you would prefer.  I am not saying that your soon to be spouse?? should be making decisions on a regular basis that are going to impact your life without your input.  But, occasionally, he may have to "decide on the fly" or there may be situations were changes in his custody could be unavoidable to a certain extent.  Note... I said occasionally.. this shouldn't be his default mode of operation.

I mean, things come up... the kids were supposed to go home Sunday night.. but his EX's father falls ill and they go to the ER and his EX needs to go for that emergency reason.. well.. maybe the kids don't go Sunday as planned.

but that should not be a regular occurance right?  

And I guess it does depend upon the nature of these decisions he is making and how they impact you.  If he has his kids for more time.. you should not necessarily be impacted there.. it shouldn't be on you to care for them.  Financially.. also, your money shouldn't be obligated for his child either.

So, maybe understanding what kind of decisions you mean here.. we could give you more specific advice.

I know most have said he should not be ever making decisions that impact you... but we all know in life, sometimes we can't always have a quorum of decision making.  My DH might make decisions that impact me and vice versa and generally, we should be able to trust that our partners are taking our interests into account when that has to happen.  If he isn't.. and you can't discuss the fact that it is a problem that he is ignoring the impact on you.?   rethinking or delaying marriage until you both have some common understanding and boundaries in your relationship.

I mean... if it is that he is getting his child an hour or two earlier.. certainly that should normally not be something he has to have a family meeting with you about.. that is fairly minor adjustment and unless he is aware that there is a specific reason that will super inconvenience you.. he shouldn't need to run every minor adjustment by you... but making plans for weeklong visitations without your input or checking on your availability or plans?  when he isn't under the gun to make that decision.. if he isn't able to work with you there.. he is probably not the right partner.

Your kids are grown and launched. Are you SURE you want to marry a guy with a 9-year-old? A lot of men expect women to take over parenting their kids, make sure that's not the case before you tie the knot.  Yes, you should be consulted on things, but ultimately, all the parenting and big decisions should fall on him.

OP, you are in the honeymoon period of the relationship.  If you don't fix this now, it will get worse once you get married.  Set your boundaries and be firm.

As tog says you shouldn't be impacted by your SO's child.

Communication and natural consequences for this behavior needs to be used here.

Your DH decides to take an extra weekend with his kiddo and you planned a nice day out doing things with him? Swap out the husband for a friend. Your plans shouldn't change just because he made a decision to change them.

Communication needs to accompany this, too:

"DH, I understand that emergencies happen and sometimes your plans change because of SS. However, I no longer feel like a priority when you make decisions that aren't emergent without asking me, either. You deciding to make changes will not stop me from doing what we/I had planned when they are not discussed and decided with me first. I will never prevent you from spending time with your son, but you have to recognize that your son and I are BOTH priorities and responsibilities to you, and you can't go all-in on one of us while ignoring the other. There is plenty of room to compromise and find solutions, but you have to extend that courtesy. I won't drop plans entirely to be at your beck and call. If that is what you want, you'll need to find a different partner."

If you don't want to do your own thing and you want to be able to rely on your partner, and your partner makes themselves not reliable, then you may need to reconsider your partner.

There is a caveat with all of this. There WILL be emergencies that wreck your plans. There WILL be a need for you to compromise. That comes with dating/marrying anyone, especially someone with kids. There are ways to reduce/combat some "emergencies" (like if BM just randomly drops the kid off on Fridays, making it a point to either not be there or your DH filing for full custody) that your DH needs to get behind. But having *some* flexibility is necessary.

How does that work?