How do I tell my son that his girlfriend’s reputation is not what he thinks?
If you approach him directly, he won't likely listen for one of two reasons. Either your relationship isn't close enough for him to listen to you, or he is in serious denial. You can leave little "hints" around, but be careful not to have it come directly from you. If you are close to any of his friends, ask them if they know her and then ask them if they think your son might want to know. Good friends will protect him, but only if he let's them.