So how does his IEP address his dysgraphia and ADHD/Executive function deficit?

Picking a few examples from my DS's IEP that might be applicable -
Instructional Accommodations:
- provide extra time to complete written assignments
- reduce the amount of writing when necessary
- scribe for written assignments

Assessment Accommodations:
- verbatim scribing of responses
- negotiate written assignments; reduce length or increase time
- assess in a variety of ways
- allow for adequate response time, oral or written
- extended time limits to complete tests

Now TBH most of the above are not always needed for my DS but if I was in a situation where his grades were being impacted I'd be pushing back really hard on them to follow what is in his IEP. Hopefully your DS's IEP has some clear accommodations that will help (and if it doesn't then I'd be looking to revisit the IEP).