Unless your baby bullfrog is exceedingly tiny, wingless fruitflies will be way too small for him.

I did some bullfrog tadpoles in the very distant past, and they went straight to full sized adult crickets. That same summer, I gave my 13 year old sister a bullfrog tad, which she kept in her goldfish tank. Even though he was very much on the small side, all the goldfish in the tank disappeared in one day, practically before the froglet had even begun to absorb his tail.

From what I can recall (and this was nearly 30 years ago) young bullfrogs are voracious eaters. My own froglets even had a go at the newts that they shared a tank with (I didn't know about not mixing species back then). They only spit them back out because they tasted bad. All of the fruitflies in one culture wouldn't make more than one or two meals for one, unless he's just exceptionally tiny for a young bullfrog. Even if he is really that tiny, small crickets would probably be a better bet for him.

As far as wingless fruit flies go, they can't fly, but they can climb, and quickly. If you don't have a cover on your tank with exceedingly fine mesh, they will escape en masse. Another reason why crickets are preferable.