What's wrong with "they are waiting somewhere for him"?
Adverbs of location are normally put after the direct object of the sentence. I don't know how common are exceptions from this rule.
Honestly it's fine to move it, at least colloquially. I put "They are somewhere waiting for him," which changes the emphasis slightly (from "waiting" to "somewhere") but is very natural.
I put that as well.I am English and for me they are the same