Hi, I'd like to help you with your question.
It sounds like your fiance's son is acting very normally in response to such a big shock. To just discover that the man he knew as his father his whole life is not really his biological father has to be overwhelming. The reaction you described Erik having sounds within bounds of normal mental health behavior. His fears of thunderstorms and being kidnapped are exactly what you thought, round about ways of coping with what he is feeling. His world has been altered and he is probably feeling insecure, fearful and unsettled. So instead of dealing with the situation head on, he is transferring it to things like storms.
I agree with you and your fiance that Erik needs to talk to a therapist. This revelation is a lot for anyone to handle. A professional can help guide him through so he learns to express his emotions about this appropriately and help him to focus on the situation and not to develop fears over other things.
Erik can see any therapist that has either a Master's degree or a Ph.D. A Master's level therapist will have either M.Ed (such as I do), MA, MSW or LSW after their names. Some have additional licenses, either national or state level. The type of degree tells you what the therapist is trained in. For example, I have a M.Ed which is a Master's in Counseling. An MA has a Master's in psychology theory and a MSW/LSW has a Master's in social work which is the study of family and social systems. A PhD is someone who has a doctorate in those fields, mostly counseling and education.
A psychiatrist is a medical doctor that specializes in mental health related medications. Unless Erik is having trouble with his daily activities and his feelings are too overwhelming for him to cope, he most likely does not need medication.
Any of the above can do an evaluation on your fiance's son and provide therapy. You may also want to consider searching specifically for a child therapist, though all therapists do have training in children and adolescent therapy. Another source to help you find a therapist is Erik's pediatrician or you can search on line at http://therapists.psychologytoday.com/rms/ or http://www.therapistlocator.net/.
Let me know if I can help in any other way.