I am assuming you took him from a local body of water? If so, please put him back immediately. Not only is it illegal in many states to collect native herps without a permit, it is not doing the animal any favors. What sort of setup do you have? Do you realize that all sorts of turtles need very large, well filtered enclosures with both land and water? That they need lightning that provides them UVB for proper shell health, and those bulbs can be expensive and need frequent replacement. Very few people actually provide proper care that turtles require in captivity, neglecting many of their needs.
Diamondback terrapins eat marine invertebrates, such as snails, muscles, shrimp, etc.
They are also considered threatened, because they were hunted almost to extinction for food.