I'm not a legal expert and I'm riding the fence about separation and/or divorce, but I felt compelled to chime in here and I hope you don't take offense.

You've said that your parenting plan states that you and your ex should give each other notice if there is a change in visitation, but does it say that the other one has to agree to that modification?  If it doesn't, it would probably be a good idea to do one of two things:

1.   Take your son on Memorial Day and give his father WRITTEN NOTICE that you appreciate his willingness to modify the plan to accommodate you at that time...AND that you are willing to accommodate him as well.  You should tell him that your (yours and his) son can stay the extra days on 4th of July weekend in an effort to maintain harmony and to give him equal rights to a child that BOTH of you share a vested interest in raising to become a productive citizen.  And you should probably consider taking your personal feelings out of it.  It's not fair to think of a reason why YOU can change things, have your son's father just ACCEPT it and not have to be as responsible to keep the stipulations of the parenting plan as he is.  It's only fair.  If you get Memorial Day, he should get those extra days for the 4th of July to accommodate the days he'll miss to allow you modify things.  

I fail to see what the big deal is.  I don't know the particulars of your situation, but I do know that if your (yours and his) son is visiting his father for a week anyway, a few more days WILL NOT MATTER. 

-OR-

2.   If you're SO concerned about your (yours and his) son spending those extra days with his father, keep the parenting plan and let his father have his scheduled time with him on Memorial Day.

I guess what I'm trying to say is that you should worry less about the legality of the situation and concern yourself more with what is fair and equitable.  If he's willing to compromise with you, you should willing to do the same thing.  After all, it's not about you and him...it's about your (yours and his) SON.  So what if your (yours and his) son will be away from you for a couple more days?  If anything, it's giving him some much-needed (and probably wanted) time to bond with and build a relationship with his father.

As mothers, isn't that our primary obligation?  --to be an integrious as we can and to be the ultimate example for our children?  --to set the precedent that what happens between adults has nothing to do with them...and that playing petty little power games with each other is NOT the way to go?  --that fairness should be considered regardless of our personal feelings about the person we're trying to work out an arrangement with?

Don't do it for YOU.  Do it for YOUR SON.  He'll remember how you handle things in the future and he'll have one of two reactions:

* He'll either resent you for what he will view as trying to hamper the relationship with him and his father;

or

* He'll look at you like the second coming of Christ and think Mommy walked on water when he was growing up because she always encouraged a relationship with his father.

Kids remember... remember that.