Children should heard, seen and believed.
Something IS going on with him that needs to be discovered. You are doing the right thing by taking him in to the Doc. If the doc is a pediatrician, he/she may be able to get "the secret" out of him. My kid's Doc and staff are all geared to interviewing kids for these "secrets" every time they see them. They do that by default with all patients
The fact the he developed a fear of the night, does not ness mean that the night is the source of his fears. It could also be daytime events that manifest in nighttime anxieties. (I know this personally).
My fault? How's this my fault? [Dean Vernon Wormer, 1978]