Hard to tell scale from the pics, but he doesn't look tiny. My adult clown doesn't swim around a whole lot (the juvie one on the other hand hardly stays still) , maybe he's just used to sitting around in his anemone. Was the anemone he had on the sand? He's still pretty new to the tank. If he doesn't look like he's breathing heavy and he's eating, he should hopefully come around when he's more comfortable. He might just be deciding if the tank is a safe place or not. Just make sure to see if he's eating and keep an eye on him.