Hi Katharine, and welcome 🙂
Dark chocolate is good as a treat - best to check the carbs on something he'll eat, but I get Green & Blacks 85% cocoa and a couple of squares of that have very few carbs (obviously sugary milk chocolate is not such a good idea!). Sweets generally are best avoided, but one or two Tic-Tacs are pretty much carb-free. Nuts are good if he is OK with those, and I also eat plain TUC biscuits, which have 3g carbs each. Cheese is fine if you're not vegan, and mini pots of Petits Filous are pretty low in carbs too. I'd avoid dried fruit, as it sends blood sugar up very quickly (I sometimes use it to treat hypos), but if you can eventually get him onto things like tomatoes, I agree with Hobbie, they are a good idea.