There are definite differences between medical and school speech services. This is the set of norms used by most school systems. I would bet your kid won’t get services or will get an intervention program that isn’t a true IEP.
https://www.veipd.org/main/pdf/webinars/readable_iowa_nebraska_norms.pdf
Both my kids went to speech, a lot is done at home with homework and correcting which is good. You just need to know the way to prompt where to place tongue/mouth to help at home. I would pay for the SLPA. If no one is truly understanding him at all, he is more likely to get speech through school, so definitely hammer that home and ask for an Eval right away. Write a note saying you are requesting to meet as a team and want testing for speech asap. You have the right as a parent to request testing and you may need to be firm to request.