I don't know if anyone is to blame here, I think you just have a normal kid.  I have an 8 year old and he just recently reached the age where he thinks he knows everything and will argue with me and DH about trivial things like you spoke of. I think it's just the "age" and he will most likely move on to some new annoying phase soon.  I think it might be harder for you because you started later in the game.  Like I realize this is a phase with BS8 because I've been there for all his phases. 

BS8 just this year started rushing through some of his school work (spelling/writing mostly) because he doesn't like it.  Again, I think that is normal for boys this age.  BS is also extremely competitive and tends to rush through his work at school, simply because he wants to be "first".  

Just continue to redirect, correct and support and before you know it he'll be graduated and in college.  

If it makes you feel any better, I think that it's not unusual for kids to be know-it-alls around that age.  They also start to be more willing to back talk and disagree with authority figures.

I know it's frustrating and it has been a long time for you to realize that your DH let his parents carry most of the weight raising his kid before you were on the scene.  Now, I'm sure he did some "play-daddy" for appearances and because he thought it would make him more attractive to you.  Not too different from the girls who try on the 'wife' hat and start to act like Suzy homemaker when they are dating a guy. 

Why he let his parents do it?  A variety of reasons.. .He may have been lazy... stuck in a parent/child dynamic with his parents..was out of town.. was being selfish and liked that he could live his life without that responsibility.

Now, with you there to guide him, he seems more willing to step up.. again.. many reasons.. out of parent's home.. more mature...job without travel and he wants to please you.

There isn't much you can do about the past.. all you can do is deal with the now.

When the kid started arguing about pesky details.. you could have cut him off by saying.  "It doesn't matter whether it was one, two or three.  The bottom line is we will try to avoid missing trips.  Your grandparents won't be here very long, so it's unlikely to interfere with many if any of them.  If it does..turn out that a trip overlaps, your dad will decide whether you need to miss it or not."

He's just being a kid. SD9 "remembers" things all the time too. It's not that she's lying, it's just that she honestly remembers it that way, and I think it's human nature to not want to be told you're wrong. Being able to admit when you're wrong is definitely a trait learned as we age.

Sorry it's been so frustrating! I get frustrated too sometimes with it!

Arguing about something not set yet is pointless. No matter what they did last year, the camp / daycare may do it differently this year. What was done last year does not always predict accurately what will happen this year.

In the short term, now and those soon to be upcoming teen years, the arguing is aggravating, but if you want to raise a child to a independent adult, it comes with the territory.

https://www.psychologytoday.com/us/blog/surviving-your-childs-adolescenc...

You are giving him too much power by arguing with him. He thinks he's at your level which is the same as what his family has done. You have to remain control. Kids this age will challenge you over and over again.

Don't fight with a nine year old. You two were fighting over something that is yet in the future (this summer). Plenty of time to work out what will be what and how any time in and out of the summer program will be handled. 

Arguing will get him nowhere except for in trouble for agruing. I use to tell my kids when they were younger that running their mouths would get them in more trouble than shutting up would. That p*ssing off the mommy was not a good idea. 

I'll assume you were partially both correct. Most weeks had two trips, but at least one week had three. Does it matter in March? I got the impression the kid really enjoyed the program last summer and the trips were especially something he looked forward to and liked participating in. Doesn't sound as if he wants to miss out on the trips this summer. Actually, though he was wrong to fight with you, it is a good sign that he seems more interested in the program and trips than taking time off to hang with the visiting grandparents. 

Grandparents are going to be around perhaps up to a whole month. Plenty of time to see them and still attend the majority of the program and all the fun stuff it concludes. Kid is not as dependent on his grandparents as he once was. He's discovered there is a social life out there with his peers that don't include them. I take that as a sign that kiddo is adjusting well to life with Dad and SM's home.

Unless it is a pre-planned 'family' day (meaning a day where everyone in the family is doing an outing or Dad and you and kid are doing a mini vacation) I'd not take him ut of his summer program at all. His extended family (Aunt and grandparents) can see him in evenings and on weekends. Let him have his normal summer routine. It'll also show the grandparents that kiddo is adjusting just fine and doing well without their use-to-be daily argument. 

As to the actual argument, don't argue with him. Simply tell the little know it all that talking back to you isn't acceptable behavior. That it can and will get him in hot water because it is disrespectful. That when summer program starts and itinerary schedule is known Dad and you will see to it that he has a chance to attend all the activities that are possible pending the family activities that are also being planned. 

The homework project? Kid knew he blew off and did the assignment poorly. So does Dad and you. Dad attempted to correct the laziness of project and kid refused. So when kid brings home the bad grade, Dad hands out an appropriate penalty. The kid had his chance to avoid the penalty. He blew it off. Let the kid learn this lesson the hard way.