Myself, I would not buy ice cream for anyone who is pre-diabetic ... it's not the same as normal sharing, where you could safely have his'n hers items.
Over on the SparkPeople site, where I've been for the past three-ish years, there are sites where people vent or ask for help with their difficulties. Probably the biggest complaint that I see is that the spouse/SO is buying/leaving tempting food around while they are on a diet. People get very stressed about this - I don't think they are making up stories when they say that they have no control. They really believe they don't, that they "have to" binge, and many become very resentful over what the other person is 'doing to them'. Personally I think control over what we eat is something that can be learned, and I agree it would be nicer if you could leave food around without having it eaten - but based on what I've observed over at SP this skill of staying out of tempting foods isn't something all people can learn overnight. Also, the individual has to *want* to learn. If your husband truly "can't" stay out of the ice cream, until he gains that skill/desire, you're going to continue to be missing chocolate lumps!
There are all kinds of things you personally could try, from laying down the law to avoiding ice cream altogether, and a lot of ground in between; but perhaps you could approach this as a couple. What does DH think? Would he be willing to learn how to leave tempting foods alone so that you can enjoy your treats? Would he be willing to try the kinds of ice cream that are lower sugar, aimed at diabetics? Do you like those? Would you be willing to enjoy your ice cream away from the house on a temporary basis while he learned to gain better control over how he eats? Or can you buy it on the day you plan to eat it, putting less stress on the will power? There's nothing right or wrong about any of these choices. Just what's right for you and your marriage, at least in my opinion, take it for what it's worth
More importantly, I'm really sorry your DH is pre-diabetic. My step mother is a full diabetic, has been for most of her life, and the number of health problems she has is horrible. My dad is also affected by having to help her in her recoveries from a diabetes - the condition once triggered a stroke, and more recently, she has two ruined feet which came about because she could not feel the hills as she literally walked them to shreds. So many activities have had to be changed because of her disease flaring up, it affected her ability to work, and the difficulties of constantly having to monitor her eating/blood levels is draining to both. It's a real strain on the marriage. Diabetes is treatable, of course ... but I really hope you can work with DH on getting his condition under control so that he will not get to that stage. Good luck to you, however you deal with this!