One of the Mods is a KP grad and a physician. I wouldn't say it's impossible, but there are certainly easier ways. For one, and I don't know Kp2001's personal story, but I would imagine he had to either leave campus to get in some of the pre-med prerequisites, or he took them after graduation from KP.
As you know, dental schools require the completion of a certain set of courses before consideration for admission. Without a bio department, KP cannot offer all of those classes.
Why did he want to go to KP in the first place? Did he want to be a ship's officer and had a change of heart? Nothing wrong with that, but I'm just curious as to what drove him to consider KP. Whatever that was, I'd just ask him if it's still there, or if he is realizing that he has other priorities. As others have noted, the academic schedule at KP is compressed and is very focused in the majors. If he is losing interest in the maritime industry and would prefer to focus on a pre-med/dent path, I think you have your answer.
On the flip side, if he is still interested in the maritime subjects and the sea, and if he still wants the adventure that goes with all of that, KP should still be in the mix. I mean, he is what, 17? He'll graduate when he is 21/22 and have five or so years to sail after graduation. If he is sick of it at that point (and once all of his obligations are up), he can take whatever extra courses he needs, take the DAT, and move on to dental school. I would think his experience at sea would be interesting to dental schools because it's not something you see on every application. If he goes that route, he's had an adventerous early 20s sailing the seas, and is ready to be a dentist at the ripe old age of 29 or 30.
No college sets you into a career field for life. A college choice is, for lack of a better term, a gamble on what you think you will like to be doing after graduation. After graduation, the reality of that career might make you want to make a change down the road. It's certainly far from uncommon to make a career change later in life.