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.

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