Hey! I can help a bit in this. Our ds was/is going through the same inconsistency!!!
1. Identify his weak areas e.g fractions or word problems pertaining to ratios.
2.Start from the absolute basic concepts with him, even when you know he has done that previously. For example with fractions, cover proper improper fractions, their addition or multiplications, how they are incorporated in word problems etc. Let him get confident in the area.
3.Solving paper is like medication, leave some gap between solving two papers and never mix them with two tough ones in a row.
4.Do some random maths before hand like time tables flash cards and then sit for the test paper. They get into the groove then
5.Because of the impending exam they have butterflies of the size of dinosaurs in their stomach. Anxiety can be detrimental to concentration. Soothe his nerves. A good foot rub or head massage does the trick along with their favorite food.
Dont worry! we all are in the same boat.