Legal Question in Constitutional Law in Indiana

You should start by complaining to whoever forced him to do it. (Teacher, counselor, principal, etc.) If that doesn't work, go up the chain of command.

But note that there might be valid reasons for this requirement. For example, if your son was in working with machinery in a shop class, the cross might have been a safety hazard. It might also have been dangerous in gymnastics. In both of these situations, the fact that it was a religious symbol would have had nothing to do with the order to take it off. But unless there was a strong reason like this, I agree that the school had no right to make him take it off.

(Please note that my answer presumes the cross was of a typical size and was worn hanging from a chain around your son's neck. If it wasn't, then I might answer differently.)