Its a couple of years since ds did NAT5 so I might get some bits confused with Higher.
RUAE -
He needs to know all his different language techniques for sentence structure, imagery, tone etc inside out and get good at finding and explaining them etc. ds created mind maps in powerpoint when he did his. I might still have them somewhere if you want to DM me you email address I can send over.
Practice as many past papers as he can get his hands on and carefully read the marking instructions to see where he could have gained extra points. DS also worked through some practice questions in revision guides - the brightred English Coursebook was good for this, but probably too late to order/work through at this stage.
Critical Essay -
ds's school got them to write essay plans/bullet points with - an introduction and 4-5 paragraphs + conclusion for each type of question - setting, character, (can't remember what other types of questions there were), memorise them, and practice manipulating them to various questions in past papers. This is difficult as he can't really mark yourself, but he should get a good feel of if he is recalling his memorised plans and could ask the teacher to have a look over a couple to see if he is on the right track.
Scottish Text -
A lot of this is similar to RUAE, it depends what his texts are, ds had poetry so he had to pull together themes across different poems, some key lines for themes, and memorise them and then practice questions.
English was also a subject ds found it helped to talk through with someone to bounce ideas off and give him some direction. That was tough as I am very much like ds - logically minded so we tried to take a logical approach to preparing for the English exam and it worked once we got started on it.
Good luck! It is a difficult subject to work out "how" and "what" to revise.