New - advice needed

I wonder what will be triggering this response, as children don't just meltdown out of the blue. Unfortunately the only way he knows how to communicate sudden pain, whether emotional, sensory or physical is through crying. In the extreme case, it could be a combination of being hyper-sensory + Monotropism, which are both collected attributes of Autistic thinkers and ADHD'rs. 

If you can describe what might be happening, perhaps we can help find a way toward prevention?

All children might be similar at this age in response to unnatural sensory assault, they won't have 30 years to dull their senses. However, Autistic children don't desensitise the same as they grow up and this becomes more clear into teens. But you could see if there are electronics with screaming high frequencies that randomly start and stop, a dog would also feel this. You could download a frequency reader to your phone. LED's are another unnatural source of pain. They don't radiate the spectrum like the sun and can actually be blinding and hazardous due to often being just a collected source of tiny lasers. There could be food allergies and gastrointestinal responses will cause sudden pain. One very big issue and still misunderstood, is the extreme difficulty with interruptions. These never change for Autistic children into adulthood, but a good rule for all children is to mind very slow transitions. 

Children also pick up on psychological discrepancies. A friend was talking about her niece melting down and after a few minutes of investigating it seems what she, as an only child was experiencing, was a type of inclusion/exclusion, with the parents treating her too much like an equal then taking away agency. Her response to being randomly given and then robbed of freedoms or adult things could only be communicated in sudden 'tantrums'. She's just not old enough or educated enough to communicate properly the inconsistencies she could psychologically perceive and feel overwhelmed by.

The meltdowns can be triggered by anything such as the wrong socks, doesn’t want to go into a shop he mainly kicks, screams and thrashes but will start taking his shoes, socks, coat etc off. All seems very sensory related. It’s not pain as he can tell us if he’s in pain and where these are more like he’s overwhelmed. He’s become distracted at meal times, has never slept well, he’s brilliant at preschool but like they’ve explained he’ll be holding it all in which is why he blows up once he’s home. We’ve found keeping him constantly engaged and interacting helps 

Hmmm. I might suggest to explore ADHD. The shared difference with Autism is usually Montropism and Hyper-Sensory, add dietary needs. ADHD kids tend to have a better command of language and a shorter sleep cycle, lifetime difficulty (or difference) with emotional regulation and seemingly 'easily distracted', but simply intake information at a much faster rate. Being Autistic, I tend to attract ADHD friends due to some core similarities which create an unspoken sense of trust and empathy. Every single one of my friends are highly intelligent, seem to be moving at a speed in another dimension, might not call for a year and then we just pick up at the same place we left off.

Montropism as a working theory combines a type of functioning which involves flow-state, a sense of everything-all-at-once (internal and external stimuli can overwhelm) and much more. https://monotropism.org/ There are now studies that suggest this can be from a biology with less GABA, which will mean anxiety is biological. I've found a nootropics blend can help and since these are everyday mushrooms like lions mane, they're no different than a staple in a diet

Being hyper-sensory, the world can often be unpredictable and without the ability to dull our senses, we feel the impact to a greater intensity - and sensory pain is pain (on the retina, in the inner ear, often these are senses we can craft but not if they're unprotected. It used to be illegal to use chemicals to gas humans! But you can go to a shop or hotel and breathe in micro-compounds which cause asthma are harmful to the endocrine system and might even cause cancer. Not just VOC's, but unnatural lights such as LEDs are painful and everywhere there's poorly made structures which create painful acoustics and sonics. I take ear plugs with me everywhere. I try to avoid unnatural clothing made from polys and nylon, 

Here's a few links which might help!

https://oneseedperfumes.com/blogs/news/world-environment-day-why-your-perfume-may-be-polluting-more-than-your-car

This interview on LEDs is excellent:

https://www.rnz.co.nz/national/programmes/afternoons/audio/2018911672/led-lights-make-many-lives-a-misery

As for emotional regulation, because things impact us without warning and the degree of impact feels like assault, preparation is crucial. Expect him to melt down and help him get the immediate response out. And walk through it with him. As he gets older, this kind of acceptance and awareness will help him to do what's necessary to take care of his needs immediately so they don't affect anyone else - such as exiting when he's overwhelmed, breathing through the impact safely away from a situation and then re-entering better prepared. As an adult, having a parent who didn't also compound these issues into further trauma but accepted your humanness and helped you to cope makes all the difference!

Thank you so much for your advice, I definitely need to read up more on the differences

Thank you so much for your advice, I definitely need to read up more on the differences