I can't help with the maths but did essay subjects at A-level, got 3 As and always finished my exam papers. Careful time management/disciplined exam technique was the way I tackled it, by breaking the time down into segments.
He will likely know in advance how many questions he has to answer and the points they are worth in the paper.
For example in a three hour exam if he has three essays to write, each worth the same score, then he should devote equal time to each question, instead of risking one brilliant essay, one average and one barely started.
So, first 15 minutes, read paper, select questions (if options are available and he has to make a choice....this might not be the case, all questions might be compulsory)
Quickly jot down 4-6 bullet points to cover in each essay (these might score him a point or two if he doesn't get finished as examiner will see the points he planned on expanding on)
Last 15 minutes of exam, aim to reread work.
Leaving 2hrs 30mins to write three essays (150 minutes, 50 minutes per essay).
If exam starts at 9am, this first step should be completed by 9.15am
Then tackle first essay 9.15am - 10.05am
Second essay 10.05am - 10.55am
Third essay 10.55am - 11.45am
Leaving 15 minutes at end to allow for a re read/a time cushion in case he runs over.
The key is knowing the times he has to hit, and keeping an eye on the clock throughout.
Wrt to his writing style, that's a different issue and I'm not sure how he gets around that.....somehow he has to be brutal and not waffle, make sure to answer the question and not just show off what he knows....it must be relevant to the question.
And if each question not equal in points, tweak time allowed accordingly....if he has had practice papers he should be able to work out what's needed.