I would like to hear a German speaker provide a translation assuming that "ihm" is translated "it" rather than "him" in this sentence. Or, if that doesn't make sense here, please explain why. I put "That will make it suddenly clear" but this answer was not accepted. Such a sentence would make sense in English.
I think both would translate as "Suddenly he realizes it" there is a subtle difference in German, but as a German speaker, I would't know how to represent that in English, probably the context would tell.
"wird" would emphasize the change of his view, whereas "ist" would rather stress clearness and finality.
"That will soon be clear to him." would be "Das wird ihm bald klar sein." With "soon" being "bald" and "plötzlich" being "suddenly". More familiar sounding, you would say "Das wird ihm schnell klar sein."
"He'll get it soon." would be "Das wird ihm schnell klar werden." or "Das wird er bald merken.", or informally "Das checkt er auch bald."
Ja, we call this "Dehnungs-h", "dehnen" meaning to stretch, and being itself an example of the Dehnungs-h. And even if there may be very few exceptions to the rule, none comes to my mind.
On the other hand you can't be sure, that a word missing the Dehnungs-h is short: "den, dem, dir, mir, wir, Hof, holen, tot, Tod, legen, Lage, wagen, sagen, Duden, Musik, ..." all with long vowels.
In case of the i, we have in addition to the Dehnungs-h "Ihnen, ihm" also a Dehnungs-e: "nieder, Liebe, siegen, fliegen".