Here is a great example of a frog tank. This is my eight gallon (this is a year old picture, it doesnt look anything like this anymore, but its still a good picture) and i had two frogs living in here, Jackie and Casper, as well as a betta. The substrate is a fine sand, and there is a lot of cover from the betta, but they could also come up and sit in the fake grass plant when they wanted to. The decor is really up to you, but they much prefer sand as a substrate, and plants at all levels of the water.
Have you tried offering bloodworms? Since he is by himself it should be much easier to judge if he is eating of not. I feed mine pellets every other day, and bloodworms (frozen thawed, NOT freeze dried, those will kill your frogs) twice a week.
My current tank setup is as follows: 1 betta, 2 ADFs, 3 albino and 3 green cories, and 5 kuhli loaches. I do water changes every third day of anywhere from 30% to 60%, and spot clean every day. need to get a picture of the tank.... lmao!