Decided and arranged: “He is being…” vs. “He will be in the office on Monday morning.”

Question

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.

Answer 1

The verb 'be' is a state verb. State verbs are not usually used in continuous tenses as they refer to thoughts, emotions, senses etc. Other common state verbs are 'have' (when it means 'own'), want, and know. You can find a list of state verbs here or you can just google 'state verbs' to find examples.

The examples you cite at the beginning (playing, going, meeting) are all actions so these verbs can be used with both simple and continuous tenses.

Note that state verbs can sometimes be used with continuous tenses, especially if we want to emphasize duration or action.

Example: I've been wanting a Ferrari for ages. (duration is important)

but some state verbs can't do this...

Example: I've known him for two years (not I've been knowing him for two years)

Concerning

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.

the app is basically right about 'he's going to do'. For 'he'll do' there are several functions - the app is only giving one of them. For your sentence

He'll be in the office on Monday morning.

the speaker is most likely using 'will' to state a future fact.

Answer 2

There are a few reasons "*He's being in the office on Monday morning." might sound wrong to you.

1. The verb "to be" is already in your sentence in the third person form, "is".
2. It's not his existence that has been decided and arranged but rather his presence.

   He's staying in the office on Monday morning.

   He's coming to the office on Monday morning.

3. Non-continuous verbs aren't often used in the "is __ing" construction.

Answer 3

Since we don't use BE + BE-ing to express existence-at-place (attendance, whereabouts, location, etc), you won't find BEING in that kind of statement, whether or not it involves notions of scheduled, decided, arranged.

He is being in school on Monday. ^{ungrammatical}

But you do find it expressing a decided and arranged event or intention:

He is being interviewed tomorrow by the New York Times.

I am being frugal next month. This month, I'm a spendthrift.

As StoneyB commented:

They are going to be unavailable on Thursday.