Step 1: find out which subjects your DS will actually be sitting for his test in your area, and whether the tests are in standard format or multiple choice. Look at your area on the EPE forum and start lurking.
Step 3: work our which topics you need to teach him in each subject. For example, verbal reasoning has 15 types of question in some areas, 21 types in other areas and more than that in a few areas. The EPE forum can help here. Some of the Maths topics will have been covered at school, others won't.
Step 4: teach him one or two topics a week in each subject. For example, one week might be ratios in Maths and compound words questions in verbal reasoning. Give him some practice questions on each topic.
Step 5: once you have covered the syllabus, give him practice questions and papers. Mark them with him, making sure he understands where and how any mistakes were made.
Step 6: Start to teach him exam technique, for example learning to read the question carefully and not spending too long on each question. Start giving him time constraints.
Step 7: in the month before the test, do some mock papers under something approximating exam conditions.
This method worked for us; twice. I only have experience of coaching my children in Maths and verbal reasoning, but in some areas English and non-verbal reasoning are needed too.