Chiming in again, sorry.
Just thought I should probably mention that with ASD related meltdowns, when we're in one, nothing can stop them. Also, it's actually highly inadvisable to touch or talk to the person having a meltdown simply because our brain goes 'haywire' for lack of a better term. Senses can become hyper stimulated, our fight or flight system is triggered and it becomes instinctive to lash out at anybody in range or to simply run away and hide in a dark corner somewhere.

Growing up I tended to favour the latter, but on my way to my room I would generally hit/punch/throw any person or object in my path. Upon reaching my room and slamming the door behind me I would scream. Nothing coherent just loud screaming like I had to get something, anything, out. Then I'd usually fall into a slump on my bed and shake/rock/cry for a while and then I'd return to normal.

It's hard to explain the feeling. From the outside it really does look like a temper tantrum but on the inside it's like your body is on fire and everybody around you is going to try and kill you, the world is suddenly against you and the noises and sensations just wont stop. Nothing makes sense. You can't process anything because your brain is too busy overloading to actually take in what's happening.

I know that can sound horrific to people but it really isn't something we can control once it starts and that's been proven, the only thing we can do is try to stop it before it starts. Sadly though with children as young as your son that can be difficult because he's too young to understand it himself or to know the feelings as they come on and take action to prevent it.

For me, I can feel my anxiety increasing, my heart beats faster and I get a sensation like ants crawling over my brain. I know at that point if I don't get out from where I am and go somewhere quiet/dark/alone, then I'm going to go into meltdown. Now that I'm an adult and know what I'm dealing with I can at least spot those signs. This may sound strange, but I have a little empty space at the bottom of my wardrobe and if I feel one coming on I go and sit in the wardrobe and close the door. The dark and silence really help.

The upside I guess (if you can call it that) is that it's not motivated by anger or rage. It's a neurological overload, and afterwards I personally feel awful. I get extremely embarrassed if anybody has witnessed my meltdowns and I feel a horrendous shame for my actions, even though I now know it's not my fault.

Anyway, sorry I'll leave you to it now, just thought the meltdown perspective from someone on the spectrum may be handy.