100% agreed he might be understating but I’m not sure the high schools change when reporting. Some may but I don’t think all. They might provide a scale.
If OP knows the schools scale for A, B, C, D - he can figure via a 4,3,2 I would not use plus or minus.
Colleges weight differently but most common seems to be to add .5 for Honors (so an A is 4.5) and +1 for AP.
Some discard 9th grade in the calc. And some discard but still review.
But most will count.