Is this considered a meltdown? How to help/cope with the situation?!

My almost 5 year-old was diagnosed with mild autism back in June 2010. Lately, he's been getting episodes where he cries and yells: "Stop! Don't cry!" and cries and screams endlessly. On one of these episodes I took him to the ER, but by the time they saw him he was perfectly fine and cooperative.

These episodes last anywhere from 5-45 minutes of just crying and screaming. It used to be he would say something, we repeated back to acknowledge we understood and that was the end of it. Now nothing seems to soothe him.

I can't pinpoint it to anything in particular--it may happen because he's frustrated with a game, because we're talking about him and his condition (such as meetings with the specialists), he wants to do something and we won't allow it; and it isn't like the autistic meltdowns that I've seen on YouTube where the child is not really looking at someone or a reaction, my son actually wants someone to comfort him but it just seems impossible to do so. I've had some success trying to distract him, but it doesn't work all the time.

Is this an autistic meltdown? How can I help him? Could it be something else? Please help, he has an appointment with the neurologist next week and I'm doing diabetes and thyroid labs tomorrow (don't know what else to do!) but I'm desperate to find a way to help him ASAP :(