How would you say 'He is waiting somehere'? Is 'hij is ergens aan het wachten' correct?
What if I want to say: He is waiting for something somewhere.? Is then this correct?: Hij is ergens, ergens op aan het wachten? Oh, and do you have make a bit of emphasis on the word 'op' when speaking, to make it clear?
Yes, that's correct. You wouldn't even need the comma. It looks funny when written down, but native Dutch listeners would automatically interpret the pronominal adverb "ergens op" as different from the first "ergens".
Why is this ergens and not iets? I thought ergens = somewhere and iets = something. Can you swap "ergens" for "iets"?
Combined with a preposition, "ergens" means something. This is an example of a pronominal adverb: https://forum.duolingo.com/comment/24981906
"Ergens op" = "op iets". In this sentence, you could also use the latter. That's not always true for a pronominal adverb, though. They're often mandatory in Dutch.