What would you call someone who had a doctoral degree in nursing? I wouldn’t think you’d be able to call them doctor as this would be confusing in the hospital setting. Is there a title used for a nurse with a doctorate degree?
You call him "Doctor Surname" unless you are sufficiently well-acquainted to call him by his given name. (I worked with someone with a DScN degree and she was called "Doctor Smith" by the physicians in committee meetings, etc.)
If you and this person will be working with patients, ask "How should I address you when there are patients present?" before the need arises. As guifa states in the comment above, the badge, or embroidered name, will show the degree, not the honorific.
Edited to add: The scope of practice of licensed healthcare professionals is set by state law, and the doctorate in nursing does not widen that scope in any state that I know about. Guifa's comment that some states, including my home state, restrict the title "doctor" in the context of health care is correct. You can expect the holder of a doctorate in nursing to know the restrictions, and to have a preference, so the best way to find out what to do when there are patients around is to ask.