"Do you know his previous boyfriend?"
Translation:Känner du hans förra pojkvän?
Can this indicate any kind of friend or does this mean he is gay?
"Pojkvän" only means one thing (a male romantic partner). Otherwise you would say "vän" (male friend, but can be used for both genders nowadays, I guess) or "väninna" (female friend).
Yes. However, it's "pojkvän".
"Du" is singular and "ni" is plural.
So if you write "ni" you are asking more than one person. If you are using "du" you are only asking one person.
In English there is no difference.
The difference happens in dutch too, but you could ask a group if they know someone's boyfriend
Yes, of course. But you can't necessarily see that in writing unless there is context.