To state the obvious, it sounds like something is triggering your DS.
Our DS is 6 now, which is a big age difference. Even so, I still think that he finds it very difficult to stop having a reaction if he is triggered by something. Your DS is that much younger, so I am not surprised that he is struggling.
Is it really as non-stop as your OP suggests?
You are going to struggle if you can't spot some patterns re what is triggering him. Classic ones with ASD are around transitions, and when there is lots going on. Remember that what doesn't seem like much to you can be to DS - he doesn't have the filters that typical people do, so try to see everything that might be overwhelming him (sights, sounds, smells, new people, toys, unfamiliar places, etc, etc, etc).
Another one is not feeling safe - not because somebody has done something bad now, but perhaps someone did once, and now there's a constant fear that anybody might hurt you at any time. This can show up e.g. in having problems queueing (frightening to have all those people behind/close to you), or interacting with other children (they're so unpredictable).
DH was pretty convinced DS had ADHD when he was that age - it's possible, I guess, but hasn't been mentioned in his assessment. It's normal to be agitated if you are very anxious. Which your DS is.
If you can figure out what's causing the stress, you will be much better able to help your DS.
I found lovebombing helped with DS when he was 3ish, pre-diagnosis. We had tried all the standard discipline techniques to no avail. I think it helped me to reconnect with DS, and it helped ensure he had lots of positive feelings.
Sorry for the mammoth post - hope some of it is helpful to you.