Has anyone ever had the issue of the printer saying that he cannot print your file that is supposed to be in grayscale when you sent it with only shades of “K” in InDesign? Thanks so much!
I don’t do print work so maybe someone will correct me on this 
but I think if you need a quick fix, you can open the finished file in Photoshop, change the mode to grayscale, and try exporting it from there.
What Peninah said should work, but it is strange that the printer says he can’t print it. Could it be that he didn’t mean that he couldn’t print it, just that he would still need to create all the color plates for printing? I would ask the printer to clarify. Assuming you saved it as a PDF, the fact that it is grayscale shouldn’t make a difference.
If the printer clarifies, feel free to repost what the exact problem is.
But as Peninah said, you can open the PDF file in photoshop and convert the mode itself to grayscale and resave, or if the file is too big reimport into indesign as a photo and then resave as a PDF…through really shouldn’t be necessary!
He said he doesn’t have a designer that works with him, so it’s a lot of work for him to change it. It would be hard to change each of the pages in Photoshop, as this is a booklet that I do once a week, around 20 pages or so…
What about creating an action in photoshop to have it convert to grayscale and playing it across your files?