Translation:Hij is ergens op aan het wachten.
How important is the word order here? Is "hij is op ergens aan het wachten" completely wrong or just awkward or even oké?
It's wrong, only the word order that is provided above, is correct.
I wrote, "Hij wacht op iets". Accepted answer. Probably bad Dutch? Maybe someone can tell me why / why not.
There are 3 pronouns (iets, niets and alles/something, nothing and everything) for which use of the pronomial (ergens, nergens and overal) is not compulsory. This means that it is perfectly acceptable to use one of these impersonals followed by a preposition.