I also moved into a more specialist area of ICT after nearly 2 decades working in support, so I can understand your situation very well. Support is such a wide field that is always changing. I often found myself in situations where I was under pressure to fix something I had never seen or heard of before, and so you develop the ability to quickly learn just as much as you need to solve an immediate problem.
That skill can transfer over to something like development and be very useful. I have managed to do what it sounds like you would prefer to - that is I have learned my new job skills whilst doing the job, incrementally. For example, I started by making amendments to existing code, so I'd Google for examples of what I was trying to achieve and by comparing that with what I was already starting with I was able to learn just enough to make the needed change. One year on, and I've built up enough knowledge to write stuff from the ground up. And my boss tells me that in his opinion I'm as competent now as anyone who has been here for years and years.
The problem is, your boss doesn't seem to have the insight to see that you could potentially learn any other way than reading an 800 page document. Seriously, I don't know anyone that could read that much continuously and retain it. You can't become a car mechanic by reading a Haynes manual. You can't fly a plane just by reading a book. I guarantee that the course your boss wants to send you on will not be pure theory, no practical.
The interpersonal problem you have is that your boss "won't take no for an answer". He has put up a roadblock to communication. The only interpersonal solution he will allow you to give him is going to have to begin with a "yes". Once you have satisfied him then you're going to get sent on the course, where you can learn properly in a classroom environment with both theory and practical elements. The course is really the "help" you say you need to ask for. By the end of that, surely you will be as competent to the level he expects. So you really need to jump through this hoop for your boss, read the document, and then you will get the course you need.
To say "yes", you've read it, you really are going to have to read it! But there are many different reading strategies. If you are studying something intensely then you read carefully and this takes time. As I said previously, I don't believe anyone can learn like that. When someone intensely studies a document it is usually a document about a field they are already skilled in - for example a historian studying a history.
Instead, skim or scan the document. Get the gist of it. Try and retain the basics involved. You can legitimately say to your boss that you have read it, and if he quizzes you on it then you should be able to give some basic answers that will satisfy him.
If he asks you a very detailed technical question that you can't answer, then you could say "Sorry, it was a very long document, I don't recall that. I'm sure the training course will consolidate what I read." This may prove to him that reading in this way has limited value, and indirectly asks for the "help" you need. However, I'm fairly confident he won't ask you anything so detailed - most managers are not deeply technical themselves, and as you say he only wants to test you on "concepts". A "concept" is really a very broad idea of something. You should be able to understand the concepts of this software by skimming the documentation.
I appreciate I have made an assumption about how technical your manager may or may not be, but the fact he believes you can learn from reading alone suggests he has not learned it himself. There are countless management theories, courses and qualifications that teach people how to manage staff - but there are also numerous counter-theories that focus solely on managing computer programmers. Many people believe that the usual management strategies do not work with the personality types most prevalent among coders. You're technically minded, and you fundamentally disagree with your manager over a point which at the core is really about personality and abilities, so I can't really see that he can be all that technical.