I've spent a lot of my working life, dealing with Change, in one form or another. More often that not it would be corporate and cultural change, but the knock-on from that is if the entity or culture are to change, the people in it need to change too to a larger or lesser degree, obviously.
I've asid it many times on this forum, but for people to change, firstly, they have to want to. Secondly, they have to recognise and answer to the “What's in it for me?” question. In a corporate sense that can be, “If I don't, I'll be sacked” or the like. I'm sure you get the drift.
In the foregoing circumstances (i.e. the person could be sacked), they usually comply with the requirement to change, albeit without grace and exhibiting reluctance – usually moaning a lot and trundling around with a face like thunder.
However, sometimes a creative approach is required. There's a phrase I often think of when I'm up against it in trying the engage change in a person. That phrase is, “How do you make a stubborn pig walk forward?” The answer is you certainly don't grab it by the ear and pull it forward, or plead with it. You pull it backwards. You pull it backwards, then release after a short while. As soon as you release the pressure they surge forwards.
If only it was that simple, eh?
But, perhaps you turn the tables and offer him lots of inappropriate food – more than he might choose for himself, and see what he does.
If he is being stubborn and just not wanting to do what everybody wants him to, you might just find he starts declining some of it, because to eat it, he would be complying.
If he laps it up, then when he's feeling all satisfied from overdoing it, you could ask him how he feels. If his numbers are high, he may go all tired and listless. But, that could be a way of opening a more fruitful conversation. Of course, that's a risk only you can decide upon.
Sometimes folks, if they're rebelling, actually get stuck in a corner. They “can't” change because it would be giving in, or conceding that “they”/you/whomever were right. Nobody likes to be wrong, and some folks are very bad at admitting it.
Perhaps if he isn't feeling like he's always fighting to get his way he might choose a better path. I'm not talking about a miraculous about face from out of control to the perfect diabetic.
You're in a very difficult situation, without doubt.
If that option doesn't appeal to you, or if you feel it is too risky, even for a week, then could you find a couple of hours to write to him? In that letter, I wouldn't advise telling him all the things he's doing wrong and the consequences of those transgressions. I would summarise it as something like, “You know you're rebelling against your diabetes,…...”, then go on to explain to him how it makes you feel. If you concentrate on how his behaviour makes you feel, he cannot tell you you're wrong. They're your feelings, so you know what they are. He might try to argue you are wrong to feel that way, but he can only do that by explaining why.
Have you considered how much longer you can tolerate this as a relationship? That's 100% rhetorical. I don't want an answer.