I just read an amazing book called "The reason I Jump" written by a 13 yr old boy w autism . He is asked questions like Why do you get so upset over the smallest thing? Every parent, teacher ,therapist should read this book it is so eye opening ....might help you to understand some of the behaviors and functions...

In our world, each request is a negotiation. If he wants candy, then he has to do something to earn it, usually try a new food, which we always keep available.

If not that, then cleaning up toys, using the potty, washing hands, or doing homework.

This does three things. First, it teaches executive functioning. (The ability to delay rewards is a critical skill needed to fit into society.) Second, it associates the reward with the good behavior. (Carrots are less scary if they are associated with candy.)

But most importantly, it builds the re-direct into the conversation. Instead of allowing him to focus only on the candy, he has to mentally switch gears to another task at hand. When the reward finally comes, it is a reward for cleaning up, rather than a reward for getting upset.

The down-side of this is that you have to be more stubborn than an autistic child. This can be like trying to wear down a diamond with your bare hands. The first two or three (or 50) times, the child will maintain his laser-like focus on the candy. If you stand firm, though, and continue to insist that it has to be earned by trying a carrot, he will usually relent. Then, the next time, it will only take 49 trials, then 48, etc...

Be firm and be consistent. Repeat the same answer and/or just stop answering repeated questions (why). I like the idea of the picture--keeps it simple (even for verbal kids): first dinner, then candy. Give him stars for multiple steps along the way. Yay, you stopped crying. You get star! Yay! You ate a bite of peas. You get a star. We've found that the more positive we do, the better.

I Firmly tell him "no candy is allowed until after we eat healthy things for dinner" (pretending that I don't make the rules makes him feel like I am not the one forcing him). Give him a path to achievement (use a picture flow chart for non-verbal kids helps....dinner then candy)...Then I make a big deal out of eating all my healthy food and looking at the chart then excitedly eating my Hershey kiss.

This sounds too easy to be true, but our childhood development specialist told us to walk away to a nearby room in earshot when there is a meltdown. (The meltdowns were at home). Often, it works! (He was in a safe place during the meltdown). He carries on for awhile and then comes into the room we told him we were going to, and hugs our leg. This was a process, it didn't happen over night.
In public, I have carried our grandchild to a safe, soft location, laid him down and said, “Im sorry you are sad. Do you need to cry for awhile?” Within seconds he stops screaming, wipes his tears and gets up for a hug.
Dont get me wrong. He still has out of control meltdowns. But I figure its a big deal even if he only occasionally stops. Worth a try?