You could also ask:
Will they be involved further?
Will they write a report / letter?
Does anyone else need to see your DS (other professionals)?
What does the EP think that the nursery should be doing to help him?
The EP will probably ask you things about your DS. What he likes doing, any concerns you have, and also if he started to walk, talk etc at the 'usual' times. S/he might also ask about any behaviour problems at home and how DS development is currently, e.g. is he toilet trained etc.
I suspect that the EP is there because the nursery have asked for advice on how to help your DS best. Depending on how things go, the EP may be back or it could be a one off visit. EPs generally don't diagnose any medical conditions so i don't think anything will happen on those lines.
I wouldn't panic too much it is a funny job title but that is all!
BTW if the EP is getting involved i would hope that the nursery have looked into an individual education plan (IEP) for something similar, for your DS which looks at any extra help they are giving him and is reviewed regularly with you there. You could ask about that and what the EP thinks should be on it.