I would be at the Education Offices at the Council on Monday morning, OP. I'm amazed you've just taken this. He can't possibly get an apprenticeship if he's got that record - you need to make a formal complaint.
The fact is that the teacher was in the playground with a cup of hot coffee. That is against all health and safety rules.
He was scalded (though he seems to be focusing more on his back) when the cup was knocked - tbh my (teaching) managers would have said that was his problem as he was the one who brought the coffee outside.
He was knocked into by a pupil - this happens every day but yes, an apology was due and given.
He said it was deliberate - yet there is no history of your son doing this and he has a good reputation (apart from lateness, which is nothing to do with this.) No witness statements have been taken from pupils.
Your son has now been suspended just as he's about to take his exams. Why would anyone do this? It's bad for the school's reputation and it will damage your son's future.
So the Education Department has to work out whether the crime was actually committed, which they can only do by asking pupils for witness statements as well as pupils, and whether the punishment fits the crime.
I would say I'd lost all faith in the headteacher at this point.