M son has a 504 as well. They are designed for children who need any type of modification or accomodation in school. If your child needed therapy (OT,PT or speech) then of course you would go the IEP route.
My guess is that he may need it for the light sensitivity, if perhaps he had restrictions on going outside for recess or if gym class were held outside one day. He would need an accomodation made for him have PE inside if all of the other kids were able to go outside for exercise. Also for the asthma, if he just needs to be watched closely then probably not, but again, if there are things that he cannot do because of the asthma and again may need a gym exercise modified for him then yes for the 504.
As far as the peanut allergy, I think it depends on how your particular school district handles the issue. In ours, we have a peanut free lunch table where at least 6 to 8 kids eat together every lunch period, all of whom have peanut allergies. A lunchroom attendant "searches" or checks each of their lunch boxes, (with gloves on to prevent cross-contamination with any other kids food they may had had contct with), to make sure no one accidently brings contraband to the table. Other than the special lunch table and a clear understanding by the childs teacher, I don't think these kids have a 504 regarding this. BUT, if your district provides no such table and your son may need an accomodation made for him, you may want to urge them to START a peanut free zone in the lunchroom. We still have 1/2 day kindergarten for most kids in our district here so for us it wouldn't be a lunch issue until 1st grade.
I DEFINITELY urge you to get moving on this not now but yesterday. Call the school, find out if the special ed director convenes 504's or maybe your school's school social worker. It is best to get on this now. You may even be fortunate enough to have your teacher assigned for next school year in on your meeting so that you don't have to repeat everything all over again in the Fall. In some school's this is difficult, they really don't like to earmark teachers in the spring, but there are lots of arguments why its better to have this person aware of your son long before the first day of school in the Fall. Make sure you tell them that you want to be familiar with the teacher and you want to give them time to research, ask questions about any of his special needs/accomodations, and it gives the PE staff, the lunchroom staff (if applicable) and the teacher time to research and learn about ways they can best accomodate your son in the Fall. It benefits the school to know all about your son now rather than the first day of school in the Fall! If you give them time, they can listen to you about the types of things he can and cannot or should not do and gives them time to find truly creative and apropriate accomodations for your son instead of just, "Sam will have to sit this one out..." Be firm and persistent with your school, but not pushy or abrasive, give them the opportunity to work with you, reasonably, then if they don't respond, it may be time to take the gloves off. But give them the opportunity to sit down with you, discuss your child's needs, and then go from there. Good luck!