There are differences between and IEP and a 504 plan. Generally speaking, the IEP includes modifications, accommodations and goals to support the disabled student's education. The 504 plan just has accommodations. Both attempts to level the playing field to provide free and appropriate public education (FAPE). A student with a disability that impact his/her learning falls under IEP. A student who's disability doesn't impact learning but does need accommodations to access the school falls under 504 (FA, asthma, wheelchair-bound etc). Food allergy accommodations can be included in the IEP if your child qualifies for an IEP due to a disability that effects learning.
I have 3 kids. 2 have IEP's for learning disabilities (LD) and my child with food allergies (FA) has a 504 plan. If your child qualifies for an IEP due to a disability that impacts learning, then it's up to the district to provide policy on how to manage the 504 vs. IEP. OCR states that if a child qualifies for an IEP and a 504 plan, then the child's accommodations for the 504 plan should be put in the IEP. Since Section 504 is covered in IDEA, then the FA accommodations can be included in the IEP since IEP falls under IDEA. Some districts find it easier to add what would usually be 504 FA accommodations in the IEP (which is fine) or have both the 504 for FA and the IEP for LD. However, many are denying FA accommodations in the IEP and that is a
violation of the student's rights. Whatever your district chooses, be sure that the FA accommodations (the ones that address equal access by making the environment safe) is addressed (written in detail).
The flip side to this is if your child's FA impacts his/her learning and is eligible for an IEP due to the FA. It would take A LOT to prove that FA is impacting the learning so much that modifications, accommodations and goals are necessary to level the playing field. If that is the case (and in some rare cases it is like a student who has severe anxiety due to food allergies & cannot function due to panic attacks, misses a lot of school b/c of exposure and complications to his/her health, all combined etc), the burden of proof is on the parent as many in the district have a hard time seeing the link between the two. If that's the case, then the child's disability will fall under OHI (as stated before).
http://www2.ed.gov/about/offices/list/ocr/504faq.html