The timing is relevant here.
If the job he's applying for represents a career change -- he's been a dentist all this time and now he's applying for a software-development position -- then most interviewers would dig into that. (I have, though it's never been as severe as dentist->programmer.) I find it's better to focus on the attractors -- "why software development?" as opposed to "what's wrong with dentistry?". You'll probably get the latter along the way, but since you wouldn't ask the question at all if he'd just left it off his resume, your interviewing will be more consistent (and fair, should that come up) if you focus on the job you're hiring for. See also the cautions in this answer; assuming you're hiring for the long-term, you want to do what you can to determine that the candidate is not just biding his time. Along the way you might be able to ask about transferable skills, but it's really up to the candidate to make the case that his years of dentistry gave him valuable skills in planning or management or whatever.
On the other hand, if the career change was some time ago -- he started out as a dentist, then switched to software development along the way, and now here he is interviewing with you -- then it's probably not that important. Chat about it if you like (anything on a resume is fair game), but remember that the purpose of the interview is to determine how the person will fit with the job you have open, so if it won't help you with that, give it a lower priority than the other questions you'll be spending time on. If you hire him you'll have ample opportunities to hear about his adventures in dentistry later. (So why did he include it on his resume? Quite possibly to explain a gap.)
If it's on the resume you may ask about it, but you are in no way obligated. In an interview it's best to focus on questions that help you answer the question "should we hire this person?". If asking about the unusual background helps, ask. If it's just a curiosity, save it for when you're chatting more casually.