Answer:
93 marks
Explanation:
If he has an average of 88.5 marks on the first four tests then the total score for these four tests is:

To obtain an average of at least 90 marks on each test the total number of marks needed for the six tests is:

So the minimum total marks he needs to obtain an A is 540.
To find the lowest average he could obtain on his next two tests to still receive an A, subtract the total for the first four tests from the total needed for the six tests and divide by two:

Therefore, he needs to score an average of at least 93 marks on his next two tests to still receive an A grade.