Weeeeell, you might try giving him a single ADF friend, or maybe two ADF friends. If what my obsessive research tells me is correct, they do enjoy fellow frogs in the tank, and interact with each other.
If you have an obliging pet/fish store, get them to maybe get a tankmate that's a simmilar size? My local pet store would laugh in my face if I asked, but if yours will do it, a simmilar size would likely be appreciated.