This is a difficult question and the meaning of the active and passive is quite nuanced. "I will tell him" would be sufficient for passing on a message from a third party with whom you do not know or do not have a close relationship with. "He will be told by me" would indicate that the message is upsetting or intimate and that the speaker must be the only one to pass the message on. For example, "Richard's work must improve or he will be fired." "Ok, but he will be told by me." It is not really a phrase that is used all the time but I hope this helps clarifies the difference.