I personally completed the selective test last year for 1st round entry into macrob last year. However, my understanding is that from now on the test will be overseen by acer and not edutest, meaning that the contents and style of the examination will differ. From what I've heard, the tests are now more related to aptitude testing/ general reasoning skills, which may be a good place to start.
For maths specifically, I would recommend starting with familiarising yourself with general mathematical concepts and knowledge up to and including the year 9 curriculum - or further - this year (hopefully he will be quite comfortable with this seeing as he is participating in a SEAL program). This will set him up to be very confident in applying these in the time of completing the exam. From that point, I would propose looking specifically into aptitude/ mathematical reasoning style questions and practice tests, such as AMC/ Maths Olympiad practice tests or similar questions.
One thing that also really helped me was participating in a maths extension/ interest group at my school, to gauge and increase my confidence in maths (which was not a strong point for me at the time).
The important thing is to focus on being confident in maths and practice reasoning through PRACTICE.
In terms of tuition, I'm not too well versed in this area as I self-studied for the exam. From what I've heard from others JAC and Hendersons were the two most popular (although they are quite expensive). I'm sure others with more experience than me can comment on their experiences with these on the thread.
I hope this can give you some insight for in your preparation and good luck completing the exam 🙂))
