word usage - 'Potential' as an adjective

Here is one of those things that I have simply never thought about until recently.

I have a friend who speaks English as a second language and so still has a few overhanging errors in his speech; One of these is to use 'potential' as follows:

He is very potential.

When I corrected this for him, I told him that in that case we would use potential as a noun, "he has a lot of potential", for example.

However, confusion then ensued as we both know that potential is also an adjective, but as far as I am aware, can only be used as a pre-modifier: potential client, potential success, etc.

Is this a unique case? Is there a reason for it?

or

Am I wrong?

Answer

It seems that 'Potential' is one of a small group of adjectives that can in fact only be attributive (part of a noun phrase). Turns out, the dictionary does indicate this:

http://oxforddictionaries.com/definition/potential

Other examples are : maximum, outright, total (in one sense), utter...