For example:
He is a reliable man inside although he looks naive outside.

a steady and dependable man/a steady and reliable man

Upon seeing the phrase "軸はしっかりした男", words like “steadfast” and “unfaltering” come to mind, but they may not work well in your context.

I think the noun "軸" and its adjective "しっかりした" may need to be "separated out" to form a more westernized interpretation for this executive’s positive attributes.

The word "steady" seems to take care of the word 軸. (“axis” is also defined as “the center around which something rotates so an axis has to be well-centered and steady.)

To that, I added "dependable" or "reliable" because "しっかりした" is defined as: secure, steady, reliable, trustworthy, level-headed (Shogakukan J-E Dictionary)

http://www.tcbreview.com/who-do-they-think-you-are.php
Eventually, my serious demeanor made people think I had leadership potential. People were willing to follow someone steady and dependable like me.

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CEO, the chief executive officer, is the principal staff person of the .... Meets expectations – The individual is a steady, consistent, dependable ...
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a man of inner strength

When Japanese says 軸はしっかりした男, it means someone with principle, do what he said will do, dependable, reliable, and trustworthy.
However, here's a nuiance that his superficial character or behavior may not show his internal quality as such. 軸は connotes that.

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Note added at 1 day15 hrs (2010-05-16 00:03:09 GMT)
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As I posted on "discussion" this contravertial 軸は should be ignored as if translated, it is only confusing. It is confusing because 軸は works as though denying what follows しっかりした男. But if you want to emphasize or really wish to convey the shade of this double-structured, split-minded rhetoric, I would suggest to add "(basically he is)" a principle man.

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Note added at 1 day15 hrs (2010-05-16 00:06:31 GMT)
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As I suggested in "discussion", 軸は should be ignored. But if you really want to convey this typical Japanese split-minded, double-structured rhetoric, I would modify my answer by adding "basically he is "a principled man.

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Note added at 1 day15 hrs (2010-05-16 00:08:20 GMT)
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Woops, I don't know why my addition posted twice.

軸がしっかりした男＝This is literally translated to "a man with firm axis", which means " a man who has a firm belief."
Now, 軸はしっかりした男：This sentence means, "Though this man may have some problem, we can say that the man has a firm belief at least." It is very difficult to make clear difference between these two sentences.


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Note added at 1 day21 hrs (2010-05-16 05:31:57 GMT)
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「軸はしっかりした男」を英語で意味合いを表現することは困難です。前後の表現をつかって意味合いを出せば良いのです。例えば、Though he looks like a naive guy, he actually is a man of firm belief.と翻訳すれば良いのでないでしょうか？