He teaches as an associate professor at XX University.hopefultoo said:In 1990 he worked as head nurse at XX Hospital.
The article is missing because its function has been taken by the adjective.
i would do this:He teaches as an associate professor at XX University.
Would you omit the article here? To me it would read as if there were only one.
No. At least in the US, these are different things; a freelancer is not an employee. (There are important legal and other differences.)
Associate professor is a title. It just isn't one that identifies the holder as unique in an organization. If it were, we would omit the article "the", which was e2efour's point to begin with. Whether one omits "the" when the adjective is used to identify the one person it describes, or whether the function of "the" has been taken by the adjective is a sort of Chicken V. Egg argument. As I see it, hopefultoo would favor one side or the other.