First, what type of clownfish do you have. Next, have you noticed your first clownfish harassing your new one? Putting two clownfish in the same tank can sometimes be problematic (or even deadly depending on the species) if you don't do it correctly.
That said, try some different foods to entice him. If you don't see any obvious spots or blotches, he's probably not sick, so there's a good chance it's just stress in the new environment or from the other clownfish. I'd try some frozen mysis. If you pick up a garlic additive, like Kent's Garlic Xtreme, that may help get him to eat as well. If that doesn't work, try live brine shrimp. They're not very nutritious, but you need to get him to start eating. Once he starts, he'll be more likely to accept other foods.
