Here is a brief roundup of some resources which may be helpful to you in understanding both IEPs and 504s, and their differences... as well as providing tips and advice on enforcing them:

1) Wrightslaw website, including
- the book From Emotions To Advocacy (FETA),
- webpage "Key Differences between IDEA and 504", from their website's section called "Ask the Advocate",
- webpage "IEP development Pop-up",
- webpage "IEP special factors Pop-up",
- webpage "IEP disputes Pop-up".

2) Understood.org website, including
- "The Difference between IEPs and 504 Plans",
- search results for IEP,
- search results for 504.

3) Twice Exceptional (2e) Newsletter, now 2e News.