Will IEP plan being developed concurrently w 504 plan undermine success of 504 plan.?

Child (10) has no mental disorder to prevent learning but is emotionally disturbed (PTSD illness-child is 3 emotionally) and has never been in school or around children other than siblings until a year ago. Child has the ability to control tantrums when taught through guardians and therapist. Desired education plan is 504. School wants IEP but child will have to be proven incapable of learning due to a mental disorder that makes him mentally unable to learn. I want child to have a 504 plan (can ONLY be done before an IEP) to see how child progresses. School wants to do an IEP and get it started concurrently w the education plan. I believe by having a concurrent IEP plan being developed will undermine the school working towards making the 504 plan successful. What do you think?