While I'm sure it must be exhausting for you, the stage he is going through is normal. What can be altered is "how" he deals w/the disappointment. I know some adults that cannot grasp this. ;)
My friend (she has a lot of kids) does is get down on one knee & explain the situation in as few words as possible. She is calm and says things like "I know you're disappointed but it's time to go do something else now." or "I know you're sad the playdate is over. We'll plan another one.)
This strategy seemed to calm them.
It explained the "why", diffused the situation, calmed them & gave them something to look forward to (the "next time").
It all took a matter of a few seconds.
I remember thinking how well it worked.
I think the key elements were:
-she was calm
-she came down to their level (heighth-wise)
-gave them hope
-gave them an element of choice (we go now, we can poss do this again)
It also calmed her frazzled nerves.
It didn't encourage the behavior.
It stopped it.
Gave him emotional support esp from mom (the most important person
in the world to a youngster).
He can pout.
It's the idea that he can't actually get his way. ex. staying longer.
The misplaced toy? How aggravated do we get when we can't find our keys? They key? We've learned by a parent showing us to put things back where we've found them so you can later find it.
The calm from you is emotional support. A hug while you expain things etc.