It is okay for you to help because your DS needs it. This is not a situation of you helicoptering because you don't like grades lower than an A, but a genuine confusion why he is failing a test he appears to otherwise understand.
I agree.
Also I do know that some textbooks will have three different levels of chapter tests. The "hard" version of these tests can be very difficult and have very little in common with what is actually taught in the book they supposedly go with. Perhaps this is what is happening? Do you know what textbook they use?
