Well, this is not something that can be "sped up."
The relationship man has had with dogs formed over thousands of years. And dogs still retain many natural instincts. Foxes are very much wild, and are read different than dogs. When a dog is agitated, it's pretty easy to read. A fox might not give any warning at all. A lot of caution and consideration needs to be taken into account. So it'll take quite some time before foxes can be considered "tame"