1 is OK; it means 'he is still CEO'.
2 means 'he is still in the company, but now he is performing the function of a CEO [which he might not have been doing before].
2 without the a means 'he is still in the company, but now he is performing the function of a CEO [as he was doing before, among other roles].
This is the way I hear it; others may have different views.
b

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