Sounds like you deserve a raise. There's a book entitled, "Getting to Yes;" one of the author's (there are two) is Ury. It's a fast read, though I don't know how late the local bookstore stays open. It is a book about negotiating; I was required to read it when I worked for a mediator. Maybe at this point in time, some of the book is online.
Write down your points and support, but also anticipate his arguments, and develop a way to refute, or successfully disprove his points.
After brainstorming your ideas, role play the conversation with someone.
Find a way to explain yourself without making it apparent that you know his "back is to the wall." Find points to raise that show how it will benefit him. If you are responsible for the finances in anyway, and know how to calculate your worth and then demonstrate that, have that prepared.
If not a raise (don't let on right away), perhaps a stipend for the extra load and training duties until the new employee is hired.
Perhaps you can ask that he shows you the confidence in doing these extra responsibilities with a new job title and higher salary.
Best of luck! I wish I knew what field you are in. It sounds interesting.