If you try to change this employee's mind, he may end up changing employers. You know what the job market is like for technical people, and unless you enjoy hunting, you are going to need to show some flexibility. The fact is that it is better to have him for a somewhat reduced period than not have him at all.
Over the near term, his teaching responsibilities will further encourage him to keep his expertise current, and this is a true benefit to your company. And, over the longer term, if he truly enjoys teaching others, he is actually demonstrating some motivations that are highly appropriate for a managerial position, and that is worth keeping in mind.
Another benefit to your company is that employees who have the freedom to pursue this type of professional activity often have increased appreciation for their regular employer. And, when these employees return to work, they typically come back filled with renewed energy and enthusiasm.
If you look at the big picture, having an employee who takes time off to do some teaching is actually a great learning experience for all involved.
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