The teacher doesn't have time to watch him or help as its a large class so she sends him into the hallway where he refuses to do anything.
I know teachers have a boatload of extra work to do, but I don't understand this. If a teacher has a student in her class who is failing math, shouldn't she be doing something about this?
I agree with previous posters who recommend trying to find out the root of your son's difficulties. And in the meantime, I wouldn't push extra math (like Khan Academy). If it's a writing problem, for example, extra practice without scaffolding isn't going to help.
If I were in your situation, I would request a full evaluation, to determine if there are any non-math difficulties that may be contributing (e.g. working memory, reading/writing issues, etc). I would also gently "test" my son to see if he understands the concepts. Does he just not understand the math, or the approach used in class, or something else.
You might try getting some recreational math resources, unrelated to what is being taught in class, to help your son realize that not all math is bad. Martin Gardner or Theoni Pappas books, Vi Hart videos, manipulatives like Zometool, these can all be fun ways of exploring math. (I just made a
post about this here.)