He quickly loses his temper, often over very simple things, which soon escalate into a full blown temper tantrum.

He is on medication at night for a sleep disorder. This is sometimes the start of the problem, when it's time to take the tablets he will start messing about and screaming an shouting at me.

He is awful with his dad and generally picks on his younger sister most of the time.

He is completely disrupting the house and I am at my wits end.

I would get a second opinion on the sleep disorder, to see if his behavior is related to a condition or a side effect of the drug.
Children often resist going to sleep and that's a problem but not a disease. Does he have apnea, snoring, something measurable and visible?

There is a tremendous amount of controversy about medications being handed out to children at an alarmingly high rate, several hundred % more than 25 years ago.
What is the drug and what is his disorder called? What were the symptoms for it?