My boss is like that. With lengthy emails he would read only so much, then call me to ask what I was trying to say. It irritated him. I look at that as my problem, not his.
It happens rarely now, as I structure my longer emails with him (and his boss) in mind: a concise summary, followed by the main content, also written concisely.
Reading the bulk of the content isn't necessary to get the idea, but it's helpful to have, even necessary, in many cases.
I also use bullets, numbers, and formatting to make the content easier to follow, to refer back to, and to digest. And maps. In my line of work (land-related issues), a good map is often an essential ingredient to an effective email.
An obvious approach, I suppose; but it took me awhile to fully appreciate how effective it can be.