Have you read the instruction of ISEE?

( I did not study it since my son takes SSAT.)

As my understanding, upper level covers grade 8-11, and your son is at 8th grade. Therefore, if he only misses a few questions, his percentile should be very high. The 满分  does not mean a student has to get all questions correct. There should be a table in the book about the score and the mistakes a students makes. (The table may not be right based on my knowledge. However, it may confirm what I said above.)

Ask your son to read through the rules, such as one mistake will take how many points away. In SSAT, one mistake takes 1.25 points away. Therefore, with 5 answers for each question to choose, I asked my son to take a guess only when he could rule out 2 answers.

Take it easy. The sky is not going to falling even your son can not get into the school you would like.