I'm not sure where you guys are located, but the grocery store should have live clams, under the name littlenecks, cherrystone, quahogs, steamers, etc. Might have to go to a seafood market. Split the clam in half, and drop into the tank. If the angel doesn't pick at it, the honest answer is that there really isn't much hope for it.
Another food is scallops. The grocery store definitely should have a bag of frozen scallops. Chop a scallop or two into small enough pieces, I'm talking bite-size for him.