You can let a few crickets roam around the tank and see if he will eat them. I have one frog that refuses to eat from my hand and will look AROUND my hand to find a cricket in his bowl. Another idea would be to put a little glass bowl in his tank for the crickets and let him eat from there. Crickets can't climb out of the bowl and you'll be able to see how many he actually ate.

You have to be careful with crickets though, you don't want to leave them in the tank with him overnight because they will bite him and cause more stress for him.

Just like Bombina Bob said, a 10 gallon should be fine for now if your frog is still tiny. Whites LOVE to climb and be up as high as they can in the tank.

Good luck and welcome to the forum! There are some great people here with lots of experience and good advise!