Hi Mattiez. Thanks for signing up.
Frogs feed almost entirely based on movement - they won't eat the pellets because they don't see them as food. Those pellets are meant for fully aquatic frogs and for tadpoles so they don't work for a truly amphibious frog like yours. Foods for froglets (baby frogs like yours) include pinhead crickets, fruit flies, spring tails and any other insect food small enough to eat but obviously they have to be alive. The easiest is probably fruit flies.
Since you live in the US, you can get fruit flies from several places - if you need to feed the frog right away I would find some locally. Some Petcos sell proper fruit fly cultures in jars that you can feed from right away. Other Petcos have just small starter cultures that have maybe 30 flies, sold in vials. I don't think Petsmart sells fruit flies but it's worth having a look.
I've ordered fruit fly culturing media from these people: ED's Fly Meat Home and I have no complaints about the products, though they are not cheap.
As for how to keep him, leopards are the big jumpers of the frog world. Bigger frogs have a tendency to crash into the walls of their terrariums and hurt themselves, so the bigger the better. If he can't jump clear across it then it's big enough for him at that size, but obviously as he gets bigger he'll need a bigger terrarium.
Keeping him like the fire belly should be fine but make sure the water is not very deep - most frogs will drown very easily right after metamorphosis (turning into a frog).
I hope this helps! Good luck.