Not sure how it is in other parts of the world, but in my country working as a professor, and having a title professor are two different things. The title is given for scientific achievements, it is sanctioned by the president, and is life lasting, like a PhD. In such case, even retired a person is still a professor. Contrary, there is a possibility to be hired as a professor by a particular university, and then the person is a professor of that particular university. So it is a work title. When one changes works, one is no longer a professor unless also given such position by the new university.
So check if the person has a life long professor title. If not, if he only had such a position for some time, it would be out of place. Imagine your CEO worked as a plumber 10 years ago. Would you call him "Dear Mr. Plumber"?
But overall it depends on what is customary in the company. You might call him "Dear Prof. Smith", and he replies "call me Bob". Or points out he's not a professor, or they don't care about titles in the company, and tells you to call him Mr. Smith. You may ask a colleague who is already working in that company, or someone who looks nice enough to sincerely answer your question.
But in the end, it doesn't matter that much. It seems you found out he was a professor by yourself, you weren't given that information from the company. So I would just write "Dear Mr. Smith".