If he's OT is suggesting a 504 plan then ask for it. I suggest that you make an appointment with the district office that manages such programs. I suspect that the teacher doesn't understand the program and/or your child's needs. The OT is the expert here and should be listened to.
Federal law requires that all school districts provide for diagnosis and treatment of all conditions that impact school success. Each district has a special department set up for this. If your son's school doesn't know the phone number call the school district office to get it.
He may not need help with reading and math but he needs help with handwriting. A 504 plan will provide the help he needs. But first the school district has to either do an assessment or accept the assessment of his OT and then they'll hold a meeting with you to decide what sort of help they can provide.
My grandson is in a school just for kids with special needs. He's speech delayed and now has been diagnosed with autism. He's on the more extreme side of special ed. He has an IEP.
My granddaughter has a 504 plan based on her inability to focus. She has ADHD. She has help with study skills and English and Math. She is not considered special ed, but her program is managed by the same office as the program for my grandson.