"That is his opinion."
Translation:Tio estas lia opinio.
For this case a simple explanation would be that tiu has to be used with a noun (it is a demonstrative adjective; and if used separately it would mean 'that one'), while tio can be independent (and works as a demonstrative pronoun), for example:
Mi studas tiun lingvon - I am studying that language Mi studas tion - I am studying that
You can read more here - the last section on the page.
(Just for the record: We may answer using tiu as well now:
Translate this text:
That is his opinion.
Tiu estas lia opinio.
You are correct
I guess it's a matter of context, though - whether you imagine a scenario with someone basically actually pointing out different options [in the case with tiu], or not [the more general, non-specific, tio]. ;-)
You wrote "That one is his opinions", except that "lia" and "opinioj" do not match up since one is singular and the other is plural. And if you want to keep "opinioj" plural, then "tiuj" has to be plural as well.
And it's an odd sentence because it sounds as if you are looking at a number of opinions and are saying that "That one is his opinion".
Usually, we don't have a small set of opinions that we look at. We just point to what he is saying and say, "That is his opinion" -- so, "tio" (that, that thing), not "tiu" (that one, i.e. here: that opinion).