As I understand it, an IEP is to help with a learning disability. A 504 is to require the school to provide certain adjustments based on physical needs so that a child can learn. I'm not familiar with 504 plans at all except on this site. Both of my grandchildren have IEPs. One is for speech difficulties and the other for ADHD. My grandson has an IEP after having been evaluated by a special office within the school district. My granddaughter's IEP was devised within her school.
I suggest that you will need one or the other in order to get the school's co-operation if they are refusing it now. Once you have an IEP or 504 the school district is required by law to follow the plan. However, as you probably realize from your own experience, being forced to do something rarely gets the best results.
I suggest that you contact the Intermediate Education Service District within your county and learn about the process for obtaining an IEP and ask them about your son and for advice on how to talk with your son's school. The IESD probably already has contacts at the school. I urge you to treat the school as if they are willing to co-operate.
After learning the language associated with IEPs and the Federal No Child Left Behind law make an appointment with the principal and have another talk. If his school has a counselor talk with her first. She is most likely to understand better about your son's needs.
Are you sure that the school personnel grab and yell at him? This is an unprofessional way to act. Could the principal be telling you that they treat all children the way that you're asking them to treat your son?
Does your son have a diagnosis? Is his condition such that he does need something different than the usual expectations of good behavior and the usual consequences for poor behavior? If so, then I'd ask the professionals at Children's Hospital to write a letter to the principal describing his condition and outlining the best way to handle his misbehavior.
When you say he has problems controlling his emotions are you referring to anger? If so, how does he express his anger? In what way can his teacher and other personnel help him to prevent becoming angry or to defuse his anger once it's in motion? Keep in mind that your son is just one of many children that the teacher is responsible for and how she manages your son may have to be different than the way you are able to manage him.
If by emotions, you mean he's depressed and easily discouraged do the same as I outlined above with a focus on depression and discouragement.
Be sure to include information for the school about what you are doing about the situation and how Children's Hospital is helping you and your son. The more that school personnel know about his situation the more likely they will be sympathetic and willing to be of assistance.
You don't say how old your son is. This might make a difference in how you handle this situation.
The most important thing that you can do to get school co-operation is to be calm, treat personnel in a professional manner expecting the same from them and give them as much information as possible. Assume the best from them, realizing that if they are not co-operative, either you or they need more information.