My wife is diabetic, but says generally, the sugar-free stuff is also high in other carbs, so it's not really much of an improvement. You're kind of asking for a low-energy food that has high-energy. For example, I hear potatoes are a pretty good energy food, but it shoots her blood sugar right up, so she can't hardly eat them.

And the scoutmaster from my son's scout troop was diabetic, but on hikes, just ate normal food. One problem he had was that it was easier to manage his diabetes if he kept to the same routine in activity and eating, but backpacking and camping messed up his routine.

Your dad might do better to run the question by his doctor. It's a general how-do-I-exercise question that should be fairly common.

Something else to consider is that nutrition is important for long rides, but presumably your dad isn't going to start off riding centuries. So for a normal half-hour or hour ride, he shouldn't really need anything (throw some sugary junk in the bag in case blood sugar drops), and longer rides are down the road a bit.