That's awesome you've him eating!
For substrate I'm a big fan of sand, with a smaller tank you wouldn't need a lot. Frogs can also swallow smaller gravel so I've heard. For cleaning it's not much different than bare bottom, your just vacuuming the top of the sand. You can buy cheap pool filter sand or even play sand, or usually pet stores carry more expensive bags of it. No matter what kind you get, make sure you rinse it really, really well lol. You can google how very easily.
I saw you mentioned a filter, but I wasn't sure what kind it is?