You have to look at why he lost marks. Identifying the why is the key to improving. Also doing more past papers. Looking at a book may not be effective for revision, he needs to be testing his knowledge.
So you use the M A R C K S system, go through the papers and on each question write down one of those letters next to every question. The mark scheme may be available either from the school who clearly have it to mark the paper or online if you know which paper he sat.
Maths error
Did you make a mistake in your maths?
Application of knowledge
You know and understand the content but couldn’t apply it to foreign situation/this question
Read the question
Did you miss a key term such as explain rather than describe because of how you interpreted the question but ultimately you have the knowledge and understand the topic
Communication
You understood the application but the wording you used to communicate didn’t get the marks. Could be specific terminology especially in biology
Knowledge
Is your knowledge weak? Do you understand the concept?
Statements
If it’s 4 marks but you wrote 2 points and you can see that even though you filled all the lines you were lacking in points
Tally up each letter to see where he needs to improve first. Hope this helps.