Hello Jupiter
You're not dim at all so please don't feel like that. We've all been there with that first diagnosis and thought what now, or at least I have.
I'll try and answer your questions in order
Epipens: the usual practice is to be prescribed 2. There are a couple of reasons for having 2. The second is there in case the first one doesn't have any effect and if the emergency services haven't got to you and there is no improvement after 10 - 15 minutes. Also, if the first epipen fails to work then the second is a backup. You would always administer the epipen and then call the emergency services immediately.
Schools usually want 2 epipens to be kept on site at all times so your GP will need to prescribe you another 3. They also usually need any other medication so Piriton, inhalers for asthma etc.
Epipen Training. I think your GP should arrange something for you as he/she prescribed this for you. It's odd to just give you a prescription and then leave you to it. Maybe if there is a nurse at your GP's surgery they could help you with this. It was my DD's allergy nurse that gave us the training when her allergist prescribed the epipens.
Referral - definitely ask to be referred to an allergy clinic. The clinic will be able to also check your DS for any of the other main allergens and will help you with the care plan that could then be given to school, any other carer etc. They also usually have allergy nurses and they can often do school visits if you need support in informing the school of the situation. Once you're on the allergy clinic list, you'll probably get an anuual appointment where they will monitor the situation.
What to do next.
Read labels on all foods and avoid anything with those ingredients. We avoid anything that has the may contain nuts or may contain traces of nuts or made in a factory that handles nuts, as DD has a high response to both peanuts and tree nuts(almonds, brazils, cashews etc.) Everyone has a different approach to this and it really depends on personal attitudes to risk I guess.
Inform the school as soon as possible. They should work with you to put a care plan into place so that they know what to do in the event of reaction. If they don't have any experience of this, then the AllergyUK website, Anaphylaxis campaign or Blossom websites all have sample protocols that you can download and use. These are just a list of emergency contact details, allergen info, medication info, plus details of what to do in an emergency. Sometimes a letter from you allergist/allergy nurse is helpful, so you might need to ask your GP to help with this.
Inform anyone else who may look after your son. So grandparents, babysitters,childminders, parents of friends who he might have playdates with etc. Be open about it and tell it how it is. You may find that you end up repeating yourself but better that than your DS being accidentally exposed.
Talk to your DS about it and if he is used to sharing foods and sweets etc with friends, maybe encourage him to not do that, esp chocolate bars as many have nut warnings on them.
The only other thing I can think of is, carry your DS's epipens and piriton everywhere with you.
There are lots of very helpful lovely people on this forum so ask as many questions as you need to. In terms of the grades of the RAST tests, I don't know what they mean as DD's results have never been expressed in that way.
I hope this is of some help. Sorry for my long reply and I hope your GP is able to help you until you get to see a pediatric allergist.
Take care