is there a comparative for quantifiable nouns and one for unquantifiable ones or we need to use weniger for both "less" and "fewer" comparative?
Because it is the subject of a (rather short) auxiliary sentence. The English version should also be "he", as per pedantic grammar.
> I have fewer coins than he has.
Well, I think the thoughts are pretty obvious: because it is a subject and not an object (Dativ). On top of that, if you are a native speaker of English you probably know that in "proper" English grammar you are supposed to say "than he (has)" and not "than him".
The nominative case "er" maintains the grammar and the logic of the sentence, because "als" is a conjunction, not a preposition. "than him" is typical of colloquial English, which puts pronouns into the accusative case when they follow anything that looks at all like a preposition.
would it also be acceptable in English to substitute "coins" for "change" or is there a different word used in German for that? In English the two are used interchangeably when speaking of money
"Coins" and "Change" aren't used interchangeably in English. "Change" may include banknotes, like "das Wechselgeld" in German.
Should be "than he," even though "than him" is in common parlance.. It sounds dumb to say "him" when the German grammar is correct.