I would put a request for an IEP evaluation in writing to your school. Many students with ADHD have executive functioning (EF) deficits. These don't always become apparent until kids have multiple classes, assignments, etc. to organize and prepare for. IEPs aren't only for academics. They can also be for things like further education, employment and independent living. If your child is even considering college, you will want to discuss how they will accomplish that with the EF deficits they have. Rather than just accommodating those, it's much better to give the child the tools and skills to be able to manage EF on their own. For example, I had a client whose high school aged son is very intelligent, gets almost all A's but has a terrible time focusing in class, turning in his homework, remembering due dates, etc. He has an IEP and we were able to schedule in a life skills class with goals specifically related to improving his EF skills.
So, again, my advice would be to put the request in writing. The school must either evaluate or give you a Prior Written Notice (PWN) with reasons why they refuse to evaluate. I would also put all your concerns and your son's needs in writing. Here's a great template. https://adayinourshoes.com/parent-concerns-on-the-iep-parent-letter-of-attachment/.