Like others I tend to use a low fat plain yoghurt with fruit. In winter I use frozen fruits, much cheaper. I don't use any sweetener, but I enjoy tart things, not sure how that would go down with a 10 year old.
A lot depends on the insulin regime he is using and when he's eating the yoghurt. If the yoghurt is part of the meal, then it might be included in the carbs for that meal, if it's a snack, then it might need to be lower.
I still feel a natural yoghurt with added fruit is more healthy for any child than artificially flavoured and sweetened yoghurts.
a comparison
Weightwatchers strawberry yoghurt
Yogurt, Strawberry (12%), Fructose Syrup, Gelatine, Modified Maize Starch, Colour: Anthocyanins; Stabilisers: Pectin, Guar Gum; Flavourings, Sweeteners: Aspartame, Acesulfame K; Acidity Regulators: Citric Acid, Calcium Malates.
Carbs per 100g :5.7g
Total 0% yoghurt
Pasteurised skimmed cows' milk, live active yoghurt cultures (L.Bulgaricus, S. Thermophilus, L. Acidophilus, Bifidus, L. Casei)
carbs + 100g : 4.0
+frozen raspberries
say 25g 2g carb
similar carb counts but less 'industrial'