If you want to ask someone how long he has been working here can we say "he is working since how long? I heard this sentence somewhere.
You basically say the correct answer in your post:
If you want to ask someone how long he has been working here
In English, this type of question is formed by switching the subject (he) and auxiliary verb (has). The correct formation of the question would be "How long has he been working here?" You can read more about this here.