Well, there's a minimal set of things you must mention to fully explain something. The only way to explain his PhD fully would be to read his PhD dissertation, as that's what the definition of a dissertation is.

Summarizing is about maximizing the amount of information conveyed per word. Saying "you have failed to explain what he does in two sentences" is a bit silly, since, if one could fully explain his PhD in two sentences, it wouldn't be a PhD.