I say get some Orange poster board, yarn and glitter and have him help you make that clown fish. (I'd just draw two matching sides of the fish, loop yarn on the top so they would hang off his arms wearable signboard style, connect the two sides at the front and back- put some goggles on his face and swim flippers on his feet to increase the cute value).
It wouldn't be a perfect costume and it wouldn't look like one out of Ladies Home Journal. But I bet your boy would adore it and it would only take an evening to do.
Costumes me and my DS1 have made together include: a dragon, a vampire bat, a king, a knight and Harry Potter.
Breeder Mama: = wife to an amazing man + mama to J-Bear (07/02) and E-Train (06/08), nanny to Little Bird (07/10).