Don't let the tail wag the dog. You paying for college will not convert a mature, frugal child into an entitled twit, nor will forcing your child to pay for college convert an immature spendthrift into the next MMM. You know your kids; they have shown you who they are over their entire lives. Is your 19-yr-old mature and responsible? Sure sounds like. So why impose artificial financial penalties by forcing him to pay for something that you can easily cover?
IMO, college is an investment in your kids' futures. You can decide whether or not to make that investment. But once you decide to make it, your attention needs to turn to how you can get the most out of it. In your case, it sounds like your 19-yr-old has some ADHD issues that mean that homework and keeping up in classes takes more of his time/energy than for another kid. At the same time, he is mature, responsible, and fiscally prudent. So what will help him more long-term, now that you've made the commitment to a college degree? Is he better served to re-learn the lesson of paying his own way, even if that means he doesn't do as well in his classes? Or is he better served to focus on doing well in his classes? Everything is a lesson; the key is to pick the one that your kid actually needs to learn.
Also: please, please treat your kids as individuals. Each one will have different needs, abilities, goals, desires, etc. (not that I need to tell you that). "Fair" doesn't always mean "doing exactly the same thing for each kid"; fair is not always equal, and equal is not always fair. So don't approach this decision as Determining The Course Of Four Kids' College Educations. Figure out what's best for the kid who's going to college in 6 months. When it's the next one's turn, figure out what's best for him. Rinse, repeat.
Finally: please note that my advice is coming from someone who was very firmly in your DH's camp; I believed kids needed skin in the game, and that paying for college would create entitled twits. Except: my own DH came from a wealthier family than my own, and his dad fully paid for his private college, and he is the least entitled person I know -- yeah, he doesn't always see the financial world the same way I do, but he works his ass off and expects absolutely nothing to be handed to him, ever. So that kinda shot my prejudgment to hell, ya know? The reality is that the parenting you do in the first 18 years has infinitely more impact on who your kids are than the single decision whether to fund college.
I've done a complete about-face. My own DD has ADHD, and I was very much worried about her getting lost and overwhelmed. So we chose a smaller private school, even though it cost more, so she'd have more personal connections and be able to work closely with professors from day 1. We chose to have her do research for credit instead of getting a part-time job, because if we're paying that much for school, why would I want to distract her from her studies for $10/hr? And you know what? She worked her ass off. She ended up with four part-time positions (some credit, some volunteer, some pay*), including being basically handed what was supposed to be a full-time post-grad job by her professor for her senior year. She is graduating in 4 years, with an engineering major and a math minor, flirting with summa cum laude, and with a job offer in hand that will pay enough to allow her to be completely independent and start saving for her own future. She went from being a spendthrift in HS to frugal AF now that it's her money. In short, she is a fully-fledged, independent, un-spoiled adult at the age of 21, ready (and determined) to go off and blaze her own way, with no parental oversight or financial support required. I could not be prouder or happier that our decisions over the past 21 years helped her get to this point -- and I do not regret one single penny I spent this past four years.
Might she have ended up here if we'd forced her to get a part-time job to cover part of her costs? Maybe. But more likely she'd have had to settle for an easier major -- that engineering and math homework took LOTS of time -- or be graduating with lower grades and fewer on-campus jobs/activities that she both enjoyed and that gave her an attractive resume to potential employers. And it sure as hell would have been a lot more stressful -- again, not great for a kids whose ADHD tends to trigger periodic anxiety spirals.
Obviously, YMMV. But I repeat: you know who your kids are, based on everything you've seen over the past 19 years. Paying for college -- or not -- will not change that. So why impose some artificial constraint to teach fiscal responsibility when (a) you can afford to cover the costs, and (b) your kid doesn't actually need to learn that lesson? Focus on the kids you have, not the lessons of your own (or your DH's) past.**
*She's the one who insisted on chasing the paid positions for her junior/senior year, because she didn't want to be on our dime more than she had to be, and by that time I was confident in her ability to handle the juggle.
**I am a huge fan of demotivational posters. I have one in my office that seems appropriate here: Tradition: just because you've always done it that way doesn't mean it's not incredibly stupid.