Well I know you said your child knew the material, so that is that. He knows it. The issue is whether the environment is right for him to show it and if the teacher is willing to see it.
Although it would be pretty impolite to imply the teacher has any issues with math concepts, do prepare yourself for that possibility. I have tutored a neighbor boy (9th grade) briefly and it was pretty clear the teacher was very bad at explaining things and frankly didn't seem to have a grasp of 'equal' and had really confused this kid (and the majority of the class). It took me about 40 minutes to un-confuse him and he did just fine on his next test and was determined to work out of the book and ignore the teacher as much as possible.
O
