Does your son use PECS at all? If so, you can use an emotions card to try to help him express his anger/frustration and give him alternatives to show his anger. Let him know it is okay to get angry and when he gets angry he can: ...(some things our son does is use a weighted blanket and has a "timeout" or pops bubble wrap or chews on a chewy tube. Hope this helps.

I think that once a melt down has started, there is very little you can do, other than weather it and make sure your child is safe (i.e. won't hurt himself). Talking has never worked for us. My son's verbal and auditory processing tends to shut down during a melt down and talking to him seems to agitate him more (because he cannot process). Spanking may make things worst: inflicting pain to someone who has completely lost it may only aggravate the situation.
I view melt downs as volcanic explosions: once they start there is very little you can do about them.
You may want to explore prevention strategies, and looking for tail signs of imminent tantrums is probably your best option.
In my experience, an ounce of prevention goes a longer way than a pound of cure. Good luck!

And you are right, he will not understand that your hitting him is any different from him hitting someone. BAD idea.

Do you know why he melts down? Is he in any behavior programs? Do you know what calms him? My son (5) is addicted to Video Games.. We have to set timers for our timers, I give a 5min warning, that way it is not a shock when it is over. I also keep rubber maid shoe boxes of fun things for floor time.. We have this right after Game Time so he gets silly mommy time and he loves it! Read up on Greenspan's Floor Tine!!

Usually a meltdown once it starts is hard to stop. Take his hand and walk him outside if possible, or down a hall. "Walk, don't talk." A child with autism who is in a meltdown can't understand long sentences, sometimes not anything. Debrief and talk about it only after he has calmed down. You can intervene before he is really in the meltdown or afterwards.