To qualify for a 504 plan your child's disability must "substantially limit one or more major life activities." It sounds like you may not be able to depend on the school to supply you with evidence that your son is substantially limited in his ability to write and/or learn. Has he been formally diagnosed with a learning disability?

Here is some good info:

http://www.ldonline.org/article/6086

http://www.ed.gov/about/offices/list/ocr/504faq.html

Read the text of the actual law here:

http://www.access.gpo.gov/nara/cfr/waisidx_99/34cfr104_99.html