Husband is currently a tattoo apprentice. He is currently struggling with black and grey tattoos, in terms of shading. Any advice would be helpful?

My husband is a tattoo apprentice at a shop in our small town. He does have an artistic background, having done all kinds of art since high school (painting with acrylics, oil, oil pastels, mixed media) his style is folk/outsider art that’s always been lush with color. Currently during his apprenticeship he is struggling to get the shading and tones correct with black and greys. Any advice, resources, books, videos on how to overcome the struggle would be helpful. He wants to learn as much as he can without relying on his instructor too much. Thanks for reading. http://bit.ly/2SmnWxw