I'd try feeding it small crickets, maybe mealworm or waxworms? I know that mealworms can be bad for other frogs like pacmans because of their chitinous outsides but they might be ok for leopard frogs, not positive. If you really wanted you could probably order some wingless fruit flies for him, I'm sure he'd eat those. You can order those online, as well as pretty much everything else. I guarantee he'd chow down on some Pinhead crickets, if the store doesn't have them plenty of sites online will. That's probably your best bet. Rule of thumb it seems with frogs is "if it fits between their eyes, they can eat it".
He might be able to eat the smalls from the pet store, just remove the hind legs of the crickets, so they are easier to catch and eat.
Welcome to the forums, hopefully you find this helpful.
