He is likely in a spiral of self loathing and depression, but he can get out of it. The best way to approach it is calmly and in an understanding way. As much as you want to shake him and shout and scream, don’t bother, it wont go in.
The sadness is probably the guilt and also coming off it, its a pretty crap mental state to be in.
If you think he can stop and you still want to be with him then you have to support him, and not keep dragging up his behaviour, because he can’t do anything about that now, all he can do is be a better person again, free of coke and to start living. He is probably also thinking about all the money he has wasted, its not a cheap addiction to have.
I dont know how coke affects diabetics. My husband was using coke and vallium to come down in a pointless effort to hide it from me, so he had two addictions to work on. It was a crappy few months, but he did it, and its like living with a different person now.