Various corals often work. I've had good luck with toadstool leathers, some of with even resemble anemones when fully open, but I recently had to give away mine, as it was overwhelming the bioload in my tank (I have a 34G Solana). Clownfish have also been known to use various Euphyllia (i.e., frogspawn, torch, and anchor corals), mushroom corals (but don't put use Amplexidiscus--they eat fish!), other leather corals, zoanthids, anthelia, Xenia--even hair algae. The simplest thing to do, though, might just be to add some more live rock and form some little caves and nooks that he can hide in. If he feels like he has more cover, he'll probably settle down, especially if he's captive-bred (and therefore not used to the protection of an anemone).
He might also like a companion. IMO, clownfish seem happier in pairs--they usually live in pairs or groups (however, groups are not a good idea in aquariums) in the wild. It's also fun to watch them interact.