I hate to disagree with Babycakes, but I'm a long-time diabetic and you need to know that agave nectar, while deemed more "natural" (like honey) than processed sugar by some-and touted as therefore okay for diabetics - needs to be consumed very carefully by anyone on a diabetic diet.
Agave is very high in fructose- a naturally occuring simple sugar- one agave distributor says that 2 tablespoons of agave nectar have 32 grams of carbohydrate- that's as much as two slices of bread or more than a half a cup of Ben and Jerry's Super Fudge Chuck ice cream (28 grams of carbs). And of course, the flour and other ingeredients will contain additional carbs.
While the agave nectar may be absorbed more slowly and not spike blood sugars quite as quickly as the ice cream it is still very problematic for diabetics.
I suggest buying an angel food cake -not a big mass produced cake from a supermarket- but ordering one at a bakery. Angel food cakes are probably the lowest in rapidly absorbed carbs for a diabetic- and maybe the bakery can make it with a little less sugar then they would for other customers. You can add fresh berries and maybe some non sweetened whipped cream (if your dad isn't on a low-fat diet) and have a great cake.