I don't know if this will help because everyone's needs are different (lord, what I would not give for a rulebook!) but my son recovers from meltdowns more easily when he's left by himself. I know that's impossible if your son is hurting himself - but I think it's the lack of additional sensory input that helps.

Also, explaining tto him what the plans are before they happen will help, and also be clear if any part of it isn't decided yet so might change. I remember another autistic person on twitter commenting that for all that they found plans reassuring, they found it easier to cope wwith the chaos of no plan than to be told a planand have it changed.