My school was VERY similar to yours from what you’ve said. In order to place into honors biology in 9th grade you had to have a 95 average in your science class in 8th grade. I believe I had a 93 or a 94 but they were very firm about their standards. I don’t know if your school system does something like this, but every year as students we took a standardized test that measured progress in core subjects. What eventually convinced them to let me in was showing them my test scores that placed me in the 99th percentile. If your son has similar test scores to show, that could help.