WHY there was such a difference in meaning between:

He has few ideas.
and
He has a few ideas.

Mm, I can see the problem explaining that difference in an EFL context! My answer fwiw, Bridget, is that the first version naturally stresses the key word FEW whereas the rhythm of the longer second phrase tends to stress the idEAS - thus the first stresses the lexical item of poverty of ideas, whereas the second stresses the possession of ideas.

I like your old habits btw - exchange them for nun others ;)