A very picky English teacher would change that sentence to: "Do you know about whom he is speaking?" as there are at least three things in that question that are considered less than ideal for very formal, very standard English. The onen that you found was who/whom. "Who" should be used for subjects; "whom" should be used for objects. And so, you're right, it should be "whom." But most Americans mess that up and (incorrectly) use "who" all of the time. So, if you keep reading that blog, you can know that you know English better than the writer does! :)

Although the sound similar, they are not interchangeable. We use 'who' if we are referring to the subject of the sentence. 'Whom' on the other hand, would be in reference to something 'receiving' the object of a verb. In this sentence the subject is he. The verb is seeing ---therefore the object would be 'whom' because it is in reference to what is being seen. A trick to test out whether who/whom is correct, is answer or replace that word with either him or he . If the sentence works with: him- use whom he- use who Ex: who should eat the cake ? HE should eat the cake Ex: whom is he talking about? he is talking about HIM