Brucey,
At 1.5 inches, you will need to feed him. They need to be fed phydo and rotifiers (if available) until they are at least 2". I personally feed until they are about 3". Then they are on their own.
It is a tough fact of life, but small clams do not have enough zooanthelle in their mantle to survive over the long term - no matter what amount of light you give them. They will eventually use their energy stores and perish. I feed mine every day to every other day. I place my small ones in the bottom of a cut off water bottle. Turn off the pumps, and "fill" the container with phydo from a syringe, until the water has a green tint. In about 30 minutes, the water is clear and I turn the pumps back on. I don't like the process of removing the clam, placing them in a bowl, feeding, and then placing back in the tank.. Seems too stressful for the clam to go through this everyday.