Not only he but also I am about to go.
I have a question about “dabei zu” + infinitive. Does this always mean “about to [do something]” ? Does the adverb “dabei” change depending on context or can you always say, for example, “Ich bin dabei zu es lesen”, “Wir sind dabei zu essen”, “Er ist dabei zu schwimmen”, "Bist du dabei zu eine Spaziergang machen?
In principle, yes.
But first, your word order is not correct:
“Ich bin dabei, es zu lesen.”
“Bist Du dabei, einen Spaziergang zu machen.”
Futhermore, the whole construction is not elegant most of the time.
It should only be used, if you want to stress that you are just now doing something.
You can emphasize it with “gerade”:
“Hast Du das Biuch schon gelesen?” “Nein, ich bin gerade dabei, es zu lesen.”
But in most cases, it is better to say:
“Ich lese das Buch gerade.”
A very ugly mistake, often heard in spoken German by native-speakers, is to combine it with “am”:
Please don’t say “ich bin gerade am Lesen”.
Thank you for clarifying! I’m aware of the Ich lese das Buch gerade construction, it’s good to know that’s more common.
often heard in spoken German by native-speakers
If native speakers say it often, I’m curious why it’s considered a mistake? Isn’t a language defined by how it is often spoken? Is “ich bin gerade am Lesen” slang?
It is interesting how one’s “Sprachgefühl” differs from one person to the next.
I wouldn’t say the “am”-construction is an “ugly mistake”. In fact, it is very commonly heard throughout Germany - at least western Germany, according to the link below, and also in Berlin where I live. The article even says: Laut Duden wird sie inzwischen „teilweise schon als standardsprachlich angesehen“ (it is partly considered standard language).
As for the other forms, all express an ongoing action, as the name “Progressiv” implies (so not “about to”, but rather “in the middle of”):
However, to me there are slight semantic differences.
I think “ich bin am Essen” and “ich esse gerade” are equivalent in meaning. They mean “I’m eating right now”, i.e. it is just about the action itself.
“Ich bin beim Essen” feels a little different, because it turns “das Essen” into more than just the action. It is an event that’s happening right now, like one of the meals of the day. Dinner is happening right now, and I’m participating. Of course it doesn’t have to be dinner, it can also be just you and your snack, but you make your own little ceremony out of it.
Maybe that’s why the “am”-construction is so popular. You don’t always want to make it sound like a ceremony.
Now, the last one is also different, because to me “ich bin dabei, zu essen” sounds kind of incomplete. It needs an object, something that the action is done to.
All of MRgK’s examples had an object: “es”, “einen Spaziergang”.
I’s hard to explain. I think it’s about completing something and reaching a goal. A verb alone is just an action, but if there’s an object involved, you have an “objective” you want to reach. And if you’re “dabei”, then you’re “at it” or “on it”, i.e. actively pursuing the goal.
So MRgK’s example:
Hast du das Buch schon gelesen? - Have you read the book already? (it’s like a TODO item)
Nein, (aber) ich bin gerade dabei (, es zu lesen). - No, but I’m reading it now / I’m on it. (You’re right I wanted do do this, and I’ve been procrastinating, but now I’m working my way through it)
As you see “Ich bin dabei” can be enough if the goal has been mentioned before.
I’m sorry this has become a rather long post. 
I just noticed that my explanation of “dabei, zu” is contradictory to the usage in the topic’s sentence, and also to the english translation of it. Let me try to have it all make sense.
It seems sometimes the verb can be its own goal. I this case the goal of “gehen” is to “not be here anymore”, and it is a process that you can be in the middle of. It is not instantaneous. You can be saying goodbye to everyone, be on your way to your shoes and jacket, be putting them on, be standing in the open door… During all of these actions you are “dabei, zu gehen”. Basically, the process starts with you making the decision to go, and taking actions to make it happen.
The english point of view seems to be “soon I will go though that door and be gone”, i.e. “going” only refers to the last step or the final result.
Wow, thanks so much for the extensive and detailed response! This all makes a lot of sense. Also particularly interested in Berlin usage of terms, as that’s also where I live.
Excited to have so many progressive forms to choose from.
Thank you for updating me - in the literal sense.
You are right: Language is always in progress and if enough people make a mistake often enough, then it is no mistake anymore.
But when I was a child (in the western end of Germany) and I said a sentence with the “progressive am” I was regularly and sternly admonished by parents and teachers alike with the ironic “Du bist gerade am dran am tun” or “Du bist gerade am dranzen”. and I won’t forget.