not a whole lot much else to add other than I share/shared a lot of qualities with your son, speaking plainly and hopefully w/o hubris, and hopefully my reasons to quit can shed some perspective on what might get him there
-young-ish (25)
-in shape
-successful for his age or with big dreams (med school, etc.)
-Can't remember what else you said but generally I see a lot of similarities
QED: doesn't fit the stereotype of the country bumpkin dipper redneck
A couple facts that held true for me, might shed some perspective on your son:
-Nothing my parents said at all got me to quit. That's not being a rebellious teenager/young adult, fact is a young 20-something male is (largely) making decisions for his young 20-something male self, bc he has his first taste of a paycheck, maybe an apartment, car/car loan, who knows. Point is, independence is in full swing, and the "parental influence" plummets off around this point.
-Dipping is getting pretty wide-spread so it's not as outrageous as your generation probably remembers it. I have finance bros in manhattan, to nature dudes out in denver, all who I grew up with, all from suburban manhattan, who dip. Essentially, the elite northeastern assholes are doing it now so the stigma is going away.
-There's probably some cause to why he does it, as the full blown 30-yr addiction hasn't taken hold at this point: I'm in the military, throwing in a hammer, picking up your shit and doing whatever unpleasant task ahead of you is a time-honored tradition, both because it's hard to fall asleep w/ it, and. ........ reasons/reasons/reasons, I'm mature enough at this point to know a lot of it is bullshit, but I sure had reasons
Why did I quit:
-It sucks to be in your 20s and see your teeth and gums taking a beating that they shouldn't (which I started to notice for me right around year 6 of dip), on purely vanity reasons. There's not much you can to do convince him, but he likely is starting to notice like I did (should my gums really be that receded?)
-Health reasons, in a holistic millennial way: being in shape/eating well/having your shit together is a trend that I've fully bought into. It became impossible to square the one aspect of my life that wasn't in line with all of this, with continuing to dip. Cancer in 20 years is way too long term to worry about.
-It was on my mind for a while, bc I knew the 20 yr dipper future ahead of me. I worked with a lot of guys who were in this position, and it's not pretty, and they hate that they are in it, but at 20+ years of dip, that's another ball game. I wished I hadn't started at 18, but enjoyed doing it, but didn't want to be unable to quit when I was 40. Hopefully that makes sense
-It was getting out of control: training was a tin a day but on steroids, holding a lip in for hours as I mindlessly droned about doing whatever it was we were doing, plus the bad oral hygiene that unfortunately accompanies field problems (think: camping). I came back from one extended training exercise last December, brushed my teeth, and my gums bled for the first time in 25 years, and then this kept happening for a scary 1.5 weeks before a dentist cleared me of anything serious. Then, I could drinking water with a dip in. Then I was gutting spit from pouches. I saw the next obvious step which was a level of not-in-control-of-myself that I didn't want to be in
-I got engaged to a very long term gf, so it was just all these factors together at the right time. Get my shit together, square the rest of my personality with the one serious outlier (my addiction), and be there for my wife because it was massively selfish to bring her into something that was damaging my health, and by extension, her life.
So point is, he's probably thinking about some of this stuff. In a non-nagging way (or there's no point to even doing it), see if you can ask about the above points and ask what he thinks about it to guide his thinking.
The only way to get someone to do what you want is by them wanting to do it themselves, those were the reasons I wanted to, maybe ask him about it and see if it gets him thinking.