According to my "Cambridge - English Grammar in Use" app, Present Continuous ("I am doing") is used for "things" which are "already decided and arranged". The examples are:
He's playing tennis on Monday afternoon.
He's going to the dentist on Tuesday morning.
He's meeting Kate on Friday.
Unfortunately, there is no example with the verb "to be".
Provided that "he" has decided and arranged to be in the office on Monday morning, and according to the explanation above, it must go like:
He's being in the office on Monday morning.
But this sentence sounds weird to me.
Furthermore, the Google Books Ngram search by "he's being in,he is being in,he's being at,he is being at" results in "Ngrams not found: he's being in, he's being at" or in extremly few hits for "he is being in"/"he is being at" not corresponding to my schema "to be in/at some place".
Hence my question:
Are the sentences
He'll be in the office on Monday morning.
He's going to be in the office on Monday morning.
the only possible forms in this case?
If so, then there is no possibility to express "decided and arranged" with "to be"? According to the app "he's going to do" expresses the decision that may be arranged, but may be not; and "he'll do" expresses a decision made right now.