Tests Average on state testing but Fails all classes

My son has ADD and he hates school. It causes him so much stress and anxiety and makes him feel like a failure. He is in 6th grade and has 54 accommodations. He wants an IEP but I have been talked out of it by the counselor and school psychologist because he scores average on the state testing. My son is on state insurance and I paid out of pocket to get the ADD diagnosis so we could get the 504 plan. He had 50+ missing assignments at the end of the first semester. He felt great once he had the 504 plan in place and knew he had a fresh start with all of the missing assignments being cleared out. He did okay for a month maybe but he is missing about 23 assignments now and he has one D+ and F's in the other classes.

I am trying to figure out my options. Do I pay out of pocket for further testing to prove the school wrong (even though i cannot afford it). Could the testing be inaccurate? If I ask for the IEP evaluation now I feel like they will be bias. They have their minds set on my son being lazy.

He feels so defeated and my heart is broken.

What is IEP again?

IEP is Individualized Education Plan. It's a lot like a 504. It's just more accommodations, as far as I know.
Also, I have ADHD, and I test well, but have had trouble with turning in assignments in the past. This is largely due to procrastination. Your son isn't stupid or lazy. He retains information well (I presume) and understands; sitting still and working through the assignments and papers is just difficult. I would pressure the school to give him an IEP.
Things I have found helpful are check sheets where we write down the assignment and get it checked off by the teacher, a parent, or ourselves. This helps us ADHD people stay on track and know what the heckedy heck is going on. It also reduces stress, because we can see what has been done.
Don't nag him too much about turning stuff in; that creates more stress and a sense of... being overwhelmed and not being able to complete stuff/succeed. Work with him on developing new, better work habits.
And for Heck's sake, have him use a planner.
Hope this is still relevant?