If he scores low in exams, may be his exam technique needs looking at?
Simple things like working out how much time to spend on which questions, and looking at the marks available for each question to work out what type of answer is required (eg if a question is asking for ways in which something influences something else for 3 marks, would indicate that 3 ways should be listed).
Doing questions from past papers and then checking the mark scheme will help him to learn exactly what is required in his answers - key words, structure etc. Also noting the type of question asked - Explain, describe, list etc all require specific answers.
As for revision - there are lots of methods which can all be useful depending on hte subject studied.
Mind maps can be quite useful for eg summarising individual science topics, or character descriptions in Eng Lit, or topics in history
Flashcards or something like duolingo for language vocab, or Eng Lit quotes
Timelines for history
Individual fact sheets for each geography case study.
There are lots of websites which give brief explanations of individual topics - BBC bitesize is good. You tube videos of science experiments, demonstrations of working out maths problems.