Disclaimer: I'm autistic, but have never had a full-blown meltdown. This answer is based on past experiences with overload which I felt were veering in that direction but may be off-base.
A good metaphor for autistic overload, as I experience it, is this: imagine your brain is a computer. Suddenly, one process begins taking up all the memory available and more. For whatever reason, you're not able to kill the rogue process. What happens instead is that other processes get shut down as your brain frantically tries to free up processing power, the surviving ones get increasingly slower and error-prone, and eventually everything just freezes completely. Some of the ways this may look:
Is a very good place to start, as filtering of sensory data gets affected and so all sensory input likely gets increasingly overwhelming. This means that even if sensory overload wasn't the cause of the meltdown it probably comes into play as it builds. In fact, there's a vicious cycle that can result where feeling upset leads to more difficulty handling a challenging sensory environment leads to feeling more upset leads to...
Dealing with the unexpected
In the situation you describe, I imagine your character would first and foremost get stuck, mentally. One of the classic autistic traits is difficulty changing gears if you're faced with unexpected happenings, and this is also something that takes cognitive resources he will no longer have. So he might spend a really long time on the "??? the room does not look the way I expect? The hives are broken? There are no insects flying around?" stage. And even once he manages to come to the conscious realisation that all his insects are dead, figuring out what to do next (as in, "what should I do other than stand here and stare") is also something difficult that requires cognitive resources which he no longer has. Long after a neurotypical character would have left the room out for blood, he's still probably searching through the wreckage confirming that yes, this is actually happening.
If he suppresses his natural tendencies towards stimming (which I think quite a few autistic people do when out in public), that's going out the window, and the stims will get stronger the more upset he gets. Rocking, flapping, echolalia, all sorts of repetitive movements, all the way to headbanging.
Even if he doesn't usually have problems identifying his emotions, it's not unlikely they'd crop up here. So he might not actually understand that he's really upset right now, and might not be able to connect the mental and physical effects of the emotion to the situation.
(There is also a vícious circle that can happen when you know you're feeling a strong emotion but you don't know what it is or why you're feeling it and this fact upsets you, which feeds into the strong emotion...)
Loss of skills
Can he usually talk? Understand spoken language? Read and write? All of those might be temporarily lost as overload progresses. Voluntary movement, too, might get more difficult and rougher/jerky.
Loss of internal narrative
This is the one that really makes meltdown meltdown for me. I have, in the past, had most of the above things happen to me, and whenever they do I can sort of feel my ability to think – to actually have a coherent narrative in my head about what is happening and what I am (probably) feeling – being eaten away. The reason I say I've never experienced an actual meltdown is that I have thus far always managed to get myself out of the dangerous situation before it was completely gone. But it is what I'd expect to see in any depiction of autistic meltdown: the character losing their ability to make decisions, to understand things, to reason, to realise that they are losing these things, even their sense of themselves – until all that remains is pain, and overwhelming incomprehensible sensory/emotional input (aka pain), and the instinctive reaction to it.
Writing that is... not easy! But I'd expect short, choppy sentences, increasingly focused on describing basic sensory input instead of lines of thought. A lot of repetition. Alienation in sensory descriptions as your character becomes increasingly unable to process what the things he is seeing/hearing/feeling signify. Alienation in emotional descriptions. Alienation from his own body as well, movements described as happening to him instead of something he is consciously doing. At the same time, it shouldn't be distanced, everything needs to be increasingly immediate and overwhelming.