Harry didn't think there was anything wrong with not being able to afford an owl. After all, he'd never had any money in his life until a month ago, and he told Ron so, all about having to wear Dudley's old clothes and never getting proper birthday presents. This seemed to cheer Ron.
(Harry Potter and the Sorcerer's Stone)
Is the construction of ‘told Ron so’ ‘verb + indirect object + direct object (pronoun)’; and ‘so’ is in apposition with ‘all’?
OED gives only adverb and conjunction in the relevant definition (effectively, In the way or manner described, indicated, or suggested).
In OP's context, "the thing told" is that Harry had never had any money in his life until a month ago. There's no need to see so as a pronoun referencing that "thing told". It's just an adverb, meaning...
in a manner embodying/conveying/signifying "the thing told"
("the thing told" will always have been already mentioned or implied by context).
Thus so doesn't refer directly to "all about having to wear Dudley's old clothes..." (but it does indirectly, since that clause simply expands on, and is a restatement of, "the thing told").
If there were other constructions where so appeared to function as a pronoun, it might be worth seeing it that way here. But I can't think of any such, so I suggest it's best seen as just another adverbial usage.
Note that to tell can be used in several ways...
But after a visit from my parents, I told. I was really feeling guilty because [blah blah].
(intransitive usage, no object)No, that's not the way it was. I told the truth.
(transitive, direct object = the thing told)He asked me what my mission was and I told him. (there are lots of told's in that link!)
(transitive, [in]direct object = the person told)I told you the truth I heard from God
(ditransitive, with the two objects what was told, and to whom)
It's true we can often use "I told you so" and "I told you that" interchangeably, so possibly even some native speakers already associate so with "pronoun" functionality...
"He insulted my mother, so I hit him so!"*
(where the second "so" would be emphatically stressed, and probably accompanied by a gesture)
In that last example, you could perhaps say so = this [gesture], making it a "pronoun" usage. But I think it's more straightforward to call it adverbial so = like this, in this way.
It's hard to argue with so as a pronoun there, referring to he'd never had &c and acting as the object of tell; or with all as a pronoun in apposition, a restatement of the object; so we have, in effect, a paraphrase of:
... he told Ron that, everything about ...
And yet I'm quite sure that that's dead wrong.
So is an adverb with a wide range of meanings, all roughly equivalent to in that manner; and tell ... so is a fixed phrase meaning speak ... to that effect.
And since all here can be omitted without materially affecting the syntax, I think it has to be parsed either as an adjective modifying the about having ... prepositional phrase, or as a compound preposition, allabout—which is certainly how it is pronounced.
So I think what's involved here is a use of tell in a double sense: first intransitively, to carry the told him so idiom, and then recategorized to carry the tell him about idiom. My parse would involve an apposition of two verb phrases, with an ellipsis of the repeated second verb:
... he told him so, [told him] all about ...
But I think what we really have here is an instance of the language running so fast it tramples grammatical analysis into the mud.
