I have no experience with Autism Spectrum Disorders (ASD), but my, now, 6 year old was a tantrumer. He has grown out of it, thank dog.
Anyway, the only thing I can suggest is 1) don't reason with him, don't feed into the tantrum, just ignore it as long as he isn't harming himself or anyone else. They aren't logical at that point.
The other thing I found helpful is to let him know what to expect before it happens. My friend has a son who has Autism Spectrum Disorders (ASD) and she explains a lot before hand and uses electronic devices as timers so he knows what to expect. Just an example: Minecraft. Tell him he is allowed to play Minecraft for 1 hour (or whatever) explain to him that the 1st timer alarm to go off means that he has 5 more minutes to play. The 2nd timer means he has to put the game away and do whatever. (she uses 2 different alarm tones on her phone) He seems to do well when he knows what to anticipate. She does this for a lot of things. It seems to help with transitions.
