How to address a professor in an office setting
If you're here in the US, I'd go with Dr. You're addressing him as your supervisor at a workplace, not at an academic institution.  He's not a professor there.  His title there is probably something like "Director of Research" which is definitely not how you'd address him. But he is a Dr. and, in my experience in 40+ years in industry, that is how he'd be addressed.

But also, at most universities here in the US, lots of instructors who do not have PhDs and are not on tenure track are still called professor.  Like me.  I don't have a PhD and my official title is lecturer, but students and others commonly address me as professor nonetheless.  (And yes, it makes me a little uneasy because, to me, there is a difference and calling anyone a professor regardless of whether they really are seems to cheapen the title for those who've actually earned it, which I have not.)