I don't know if this would work for you but:

my daughter has a "cool down" jar which she shakes when she gets too upset. It's a mason jar that we filled with water, some food coloring, a verifiable ton of sparkles, and some glue. When then super-glued the lid onto the jar and she decorated the lid with stickers. When she gets too upset or can't figure out how she is feeling she shakes the jar and watches it until the sparkles fall to the bottom (it takes about two minutes). This has helped her greatly.

I have also been working with her on the idea that "it's okay to be sad when you lose something, but you should also be glad that you had it to begin with." (This applies to many things; she got upset when it was time to go home from a birthday party this last week and I reminded her that even though it's okay to be sad because you will miss your friends, it is also okay to be happy because you got to see them).

On the topic of lost things and not being sad, I'd recommend the book "Pete the Cat and his Four Groovy Buttons!" It's an adorable story about a cat who is wearing his favorite shirt (which is a button-down) but he keeps losing the buttons. The overall theme is "Buttons come and buttons go, but do we cry? Goodness no! We just keep walking along and singing our song."

Another good book on the topic of missing things is "Junie B. Jones is Not a Crook." It's a small chapter book about Junie B.; her grandfather gives her new mittens, and the new mittens end up missing on the school playground (I believe she is in kindergarten at the moment). She checks the lost and found but it's not in there, and ends up finding a multi-color click pen on the ground. She decides she is going to keep it, but then feels bad because it belongs to someone else. She ends up getting her gloves back and returning the pen. There's not a moral to the story the way there is with the Pete the Cat book, but my daughter is almost seven and loves it (and it's a good lesson on the "finder's keepers" rule).

I hope this helps!

If this were my son (almost 7), I would assume that he was having a growth spurt and insist on more sleep for a couple of days. Sometimes, I even take naps with him on the weekends if he is easily upset like you are stating. When he is calm, we talk about what the feeling was and if he can put a word to it. Then I try to empathize with that feeling. Then we talk about whether crying helped the situation or if it maybe made it worse or maybe it was because he didn't have his reserves because he is growing so much he may be tired--is there another way you could have handled it that would have been more helpful to you? If he says, "Yes, crying helped me because I was upset and needed to get that emotion out" then I will say, "That's great! I'm glad you found a solution that worked." I prefer crying to acting out or being nasty with words. It is a more accurate manifestation of the feeling of being frustrated or overwhelmed. However, usually, he says something like, "I probably could have taken a deep breath, (said goodbye to that pencil and looked for another pencil)." Then, the next time he has this situation, I can say, "Remember when you came up with a strategy with your pencil?" or "Remember last time you got really upset over something the teacher thought was not a big deal it turned out to be that it was your body's way of indicating that you needed more sleep?" I think this helps both of us because I can validate his feelings, make sure I am taking care of his body, and teach him how to be thoughtful and reflective about coming up with strategies to manage all sorts of feelings, rather than just telling him he shouldn't have the feelings. I hope that is helpful! Good luck!