My DD does a similar rage thing when she can't handle a noise stimulus. Was there a lot of noise or electrical equipment etc. around? Alternatively, what is their sensory sensitivity? Can you see if something related to that set him off? Fidgets can be wonderful tools for this sort of thing. DD always has one attached to her pocket, they make a big difference regarding their sensory processing.

Using social stories may help too.

I highly recommend doing the free courses through positive partnerships - autismtraining dot com. There's lots of little hints on there and the planning matrix is invaluable!