I know everyone says to avoid any fish related to the goldfish, but if you use them sparingly (not every single day -.-) they are perfectly safe to feed your frog. They like the smell of fish. I recommend using the rosy minnows for babies, just put them in the tank, feed them some food (maybe add some boiled lettuce for a healthier diet) and use those to feed. If he still refuses, I would usually make them angry (causing them to bite) and they would bite the food and after that would have no problem eating. All you need to do usually to make them bite is bop them on the nose with the food.

Hope this helps