When you say a serious peanut allegy do you mean that he has a life threatening reaction when there are peanuts in the room?
My granddaughter has a peanut allergy but has never eaten peanuts; therefore we don't know what reaction she would have. We told the preschool about the allergy and gave them cashew butter to use when they fed the rest of the kids pb and j sandwiches. From the time she could talk in sentences she would tell people that she was allergic to peanuts if anyone offered her any. I don't know how she knew but by the time she was in kindergarten she could taste peanuts if she did get something with peanuts in it. She'd spit the food out and adamantly reminded us that she could not eat that. That's not to say that she's never consumed peanuts. We don't know. She has had food that was prepared in a place that processed peanuts and not had a reaction. With the school nurses encouragement we are more careful about eliminating that sort of food also.
Once she was in the public school system they required that an epi-pen was kept at the school. School staff had training in recognizing symptoms and administering the medication.
I have never heard of a peanut free school. I do know of several children at my daughter's school that do have peanut allergies and the staff is careful to not expose them. Her kindergarten teacher purchased all snacks, with money that all parents contributed towards snacks, herself to be sure that there were no peanuts involved. But, now, in 1st grade, the teacher has the children bring their own snacks from home.
My granddaughter's preschool were very careful to see that she didn't get any peanuts. And all of their food was prepared from scratch in their kitchen. I think that's as close as you're going to get to peanut free. You can discuss this issue with various small preschools and may find one that will work with you.
Unless of course your son has already had anaphalatic shock from being exposed to peanuts. Or his breathing has been compromised. In that case you might want to not try preschool. What recommendation has your son's doctor made? Has he seen an allergist?
In that case you could provide socialization learning by participating in play groups. And you could "home school." There are lots of good supplies at a school supply store. There are groups that help each other with home schooling but I don't know if preschool is included in that.
Allergies certainly do complicate life. I have several adult onset food allergies and peanuts is one of them. I've read that children often outgrow food allergies. My granddaughter was also allergic to milk and eggs as a toddler but now peanuts is the only food allergy left.
I've also started well into anaphalactyc shock when stung by a bee two different times and once when I was administered a contrast solution. Two times the doctors gave me a Benedryl type of pill which stopped the process. However with the second bee sting in the same day I did receive a shot. I was 12. This was before epi-pens or any other home way to stop a life threatening reaction. I was on a farm and it was at least 20 minutes before we got to the doctor. It became increasingly more difficult to breathe and my face became quite swollen. My aunt gave me an antihistamine pill before we headed into town.
Often an oral antihistamine will stop a less serious reaction. I've given my granddaughter, at her pediatrician's recommendation, Benedryl at double the dose when she's had hives. It's surprising how quickly the hives disappear.
Hope this helps.