Autism/ASD/PDA and aggressive behaviour....help!

[Deeedeeee]

My son is 7, diagnosed ASD (autistic spectrum disorder). I have done plenty of parenting courses (some autism-specific some not) and safe handling courses. I have lots of strategies to manage behaviour, but the problem is some days nothing seems to work - he is just angry, everything seems to trigger an outburst, he gets aggressive and violent. Other days he is the opposite and can cope with all sorts of things you would not expect an autistic kid to be able to do (like noisy busy environments, spontaneity, adapting to change).
He is verbal and 'high functioning'. He wants to be in complete control all the time; he thinks he should be able to do exactly what he wants all the time (like a 2 year old, he seems unable to think about ather people's feelings). He does not respond well to being told/asked to do anything- on a bad day, even the slightest request can trigger a meltdown.
I have heard of PDA (pathological demand avoidance) does anyone have any experience of this? Or of managing an aggressive autistic child? Any tips or ideas of where I could get more support? ...Or anyone else have days when they don't make it out the house and have to cancel all their plans because they asked their child to clean their teeth?

I have heard of it. It can be useful to have an egg timer and say when the timer goes it means you have to x.
It's the library's rules. So you are not making demands.

Thanks Wolfiefan, I have heard shifting the demand onto someone/something else helps. Need to try that more and be really careful how I phrase a demand when he's in one of those moods. We use timers a lot too!