@Baldimo,
Hi there!
You absolutely should not have been dragooned into being his interpreter.
They absolutely should have provided an ASL interpreter.
I'll answer your numbered questions first.
1) I really think you should leave this to him. If the interview already happened, with you as the interpreter, probably not the best idea to request another
2) Again leave that to him.
3) All him!
4) Dunno, but not really pertinent. Deafness is its own issue.
Okay so now on to more general comments.
Disclosure is a really big issue in terms of jobs and deaf folks. Especially now that so much is done online, there are more opportunities for deaf people to be judged on their actual skills, experience etc. rather than a potential employer's foregone conclusions about what deaf people are (or are not) capable of.
Meanwhile, the pre-hiring period is one where employers can make all kinds of subjective decisions about who to hire, and it can be super difficult to pinpoint that they're making a decision because someone is deaf.
So there are various ways that can be handled. One is to bring your own interpreter, even though the potential employer is in fact supposed to provide one. This is back to subjectivity and making the best impression. (The interpreter should NOT be a family member. That's another issue.)
Another is to go ahead and request an interpreter, as you did. This is entirely appropriate. It's a little risky, especially depending on what kind of a job it is. A fast food place isn't going to provide an interpreter for every minute he's on the job, so they're looking for how he is going to do the work. Does he read lips? Does writing work well? Does he have some other sort of accommodation (speech-to-text technology, say -- not that anything is currently up to this) that he plans to use?
How does he in fact plan to perform the essential functions of this job? Will he be taking orders? How, if so? (Perhaps there is a menu that people can point to -- there are many simple and cheap accommodations.) If he's going to be doing cleaning sorts of things, maybe he only needs to have an ASL interpreter for occasional staff meetings. Etc.
Have him research what benefits there are to the employer for hiring someone with a disability (there are tax credits, for example). Encourage him to talk about that during his interview.
http://www.askearn.org/wp-content/uploads/docs/askearn_taxincentives_factsheet.pdf
He can also think about/ research possible accommodations; perhaps there are simple and cheap accommodations that they hadn't already thought of.
Generally my advice is -- let him take it from here. This specific job may not work out but it'll be a learning experience either way. Good luck to him!