I believe that 'elders' are due a certain amount of respect unless they sacrifice it through bad behaviour
Much depends on the age of the young man
Where is his mum in all this?
And his dad?
If the stepfather is a narc then the 'received wisdom' is that he will not and indeed cannot change, and if he is a malignant narcissist he could be dangerous, especially emotionally.
The young man needs protection and support, personally but also professionally, so he can understand and manage the situation. I don't think 'respect' is the most important point, I would put safety ahead of it.