I understand the MAA's position regarding higher scores on AMC10 versus AMC12 because AMC12 is much harder. It is my understanding that the two tests share 12 questions while the remaining 13 questions are easier (than the shared 12 questions) on AMC10 and harder (than the shared 12 questions) on AMC12.
Your different critieria for different grades on the same AMC8 test didn't make any sense to me until I realized that you were likely using grade as a proxy for curriculum. In our school district, an average 8th grader would be taking Algebra I whereas a GT 7th grader would also be taking Algebra I. Your reasoning didn't apply to DS because although he is a 5th grader, he has been accelerated a couple of times by the school district and is currently taking Algebra I.
DS is not a likely candidate to qualify for AIME at this point since he would have to score in the top 2.5%. However, based on the prior year's AMC10 that he took, DS has an even chance of scoring high enough to qualify for the achievement certificate for 8th graders and below, which looks like about 80 percentile or even a little lower on an easier test. 90 out of 150 is not impossible to get with just Algebra I when you also get 1.5 points out of 6 points for leaving a question blank. In fact, answering 12 correctly, leaving 12 blank and missing 1 will yield a score of 90 out of 150.
