Sugar Free Cake Mixes
Does your bil have type 1 or type 2 diabetes? If he has type 1 just make the cake normally (with sugar), show him the recipe and he can adjust his insulin dosage to account for it. If he is type 2 he probably shouldn't be eating cake for dessert, whether with or without sugar. Either way it has a lot of carbohydrates.
Check netrition.com for some possibilities. But you might ask your BIL what kind of sweetener he uses. If he doesn't tolerate Splenda, you need to know before buying. There should be some prebaked goodies at that site as well. And, in the diet section of your market, there should be some sf cookies. You can always serve fruit and cheese for dessert. A lovely soft white cheese with some of the marvelous pears in the market right now would make a lovely dessert.