Assume you have some sort of container.encloser for it? Make sure you have a dry area in it, possibly slightly elevated. Put small amount of hamburger meat, proportioned to turtle size, in the spot and leave it alone for awhile. If he is eating, he will find it. But if you sit there and watch him, he can see you and will interpret you as a threat, so he will not eat until he feels safe. Word of caution - baby snapping turtles grow up into adult snapping turtles - they don't make good pets, and can be very dangerous. Unlike other turtles, at adult size, they can extend their necks out to the point they can bite at something laying on the middle of their shell. And can do it faster than you can move your hand away. See if a local zoo will take him, or let him go in a safe environment if he is old enough not to be prey.
Just put the food in the tank and theve the turtles/turtle alone.
Make sure is has a dry spot in the tank or whatever your useing and a wet spot like a beach and snapping turtles mostly eat algea but you can buy special food for a snapper but its probably not eating cause its wild and suddenly got into captivity.
Go to an area and buy or collect dead but not rotton fish. They should get then