On a side note...May I ask if he was alone without his mother? It's just that from your post, it sounded like you have a 3 week old baby without his mother, and in a topless pen instead of some sort of cage.
If I am wrong, forgive me. And I doubt he'll stray too far away from his mother, since he still needs to nurse from her. Perhaps placing plastic bags all around where his mother is, and his cage would be a good idea as well as placing the bags underneath the food(see other thread for this advice)