Welcome to TUD, @Jandy14, this is a really great place full of very experienced people who want to help others dealing with this disease. I'm not necessarily the best person to respond on this but I can start off with one thing that strikes me as significant:
I was in my 20s when I was diagnosed so I was fortunate to miss the experience of dealing with it while still a minor, still in high school, struggling with this weird disease that marks you off as different at a time when fitting in and being popular is so critical. There are others here who can speak to this from personal experience, but a lot of people who lived through dealing with it as kids--not just the disease itself which is hard enough, but the whole social and psychological thing of going through adolescence with it--tend to be much more "closed" about it. There tends to be a lot of sensitivity to other people being aware of it--None of their business!--let alone offering criticism or advice. It also seems to be pretty common for those T1s to go through a lot of turmoil, burn-out and denial when they reach their early 20s and it stops being something that parents and pediatricians ride you about: now it's your problem. That can be a hard transition.
This all puts you in a tough corner because you are in a position where his self-care is no longer just his concern. With a wife and kid, their safety and well being are irrevocably linked together with his, and--not to put to fine a point on it--he needs to take on board the fact that he's got the responsibilities of a grown up and needs to act like one, in this area as in many others. Not that that's easy for any of us, but that's life. I like how you put it--"knowing how to support" him is the right way to think about it. But bringing up the topic at all without putting him on the defensive is the tricky thing. There can be a lot of anger hiding behind this particular wall. Being an adolescent who has suddenly been singled out for this special piece of ~~~abnormality!!!~~~ landing in his life, the gross unfairness of it, and all that, well it sounds like you're a sensitive person so I'm sure you can already imagine how disorienting and painful and angering it was.
Like I say, I can't speak from experience about what it's like being dx'd on your 16th birthday, but I do know something about successful marital relations, having just celebrated my 40th anniversary to a woman I love more deeply every passing year.
One thought is to not approach it as "There's this problem you need to fix!" but maybe just from the point of view that, hey, you've tried to educate yourself about the mechanics of the thing but that doesn't tell you anything about what it was like. This is a big part of his life's story, and it's an entirely appropriate thing for lovers to want to hear and tell those stories. Even--especially--the hard stuff, because that's what building intimacy is all about. If you can approach it as a matter of discovery and delight, not judgment or nosiness, that may lead to being able to look at how he's handling it now without there needing to be a confrontation about it. Virtually everyone with T1 has gone through times of doing the absolute minimum (or worse) out of frustration and denial, and the decision to get back into line with it can only come from within; no one else can do it for you; it's just too constant a thing. There are a lot of things no one from the outside seems to really grasp. Scary stuff like what a bad hypo really feels like--I still remember my first one, over 30 years ago, like it was yesterday. "What was that really like?" can help get across that boundary of "No, you really have no idea what we deal with."
Just talking about the experience--what actually happened, what was that like--with someone who already cares may help him unlock his own blocks and maybe initiate his own change of perspective. Has he told you the story of that day he was diagnosed--his f'r cripesake birthday no less--what actually happened? What it was like for him? Or what was the most asinine T1-related thing anyone ever said to him? Topic threads like that on forums like TUD are hugely popular, we've all got stories to share, and sharing them puts you both on the same side with respect to his T1, which is where you clearly want to be. These specific ideas may not be the right fit but you get the idea. I would just be honestly interested in this part of his story, encourage him to tell you more about it not with any agenda in mind but just because it's part of finding out more about him and sharing that stuff is part of the richness of being in a relationship. And see what develops from there.