An IEP stands for Individualized Educational Plan and is usually implemented for children who have specific learning disabilities or special needs. The first step is asking yourself if your child has a disability. If you can say there is a disability, then you need to ask yourself if the disability is causing your child to not progress in school. If you again can answer this with a yes, then you also need to ask yourself if the school can provide specialized services to help your child progress in school. Usually an IEP will directly address concerns with cognitive impairment, expressive/receptive speech, fine motor skills, and/or gross motor skills. I know there are other circumstances, also, and perhaps someone can provide a bit more information in that regard.
I would start with your son's teacher and discuss your concerns. If they also see the same issues with your child as you see, they can help you to pursue it further within your school district.
My personal opinion is that an IEP is not to be taken lightly. It's not something you just decide you want for your child and its done. There has to be a specific need for your child to have an IEP and your child's services and goals are set based upon those needs.
For instance, my son has Down syndrome, so over the years he has been in a special education program for cognitive impairment, with special services providing speech, OT, and PT. It will change based upon your child's abilities and how they progress and will be evaluated each year. My son is still in a special education class, but has progressed to partial inclusion, and only OT services. The goal is to place your child in the least restrictive learning environment possible to provide your child with the optimal learning environment for them.
You may want to post on the disabilities board for additional information.