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. Marianne

One of the main reasons the passive is used is to shift the focus of the subject. The man set the house on fire. The house was set on fire by the man. It is also used in writing and speaking to vary what is being said. The man set the house on fire. The man walked to get water. The man brought the water back. (now vary the conversation) The water was used by the man to put out the fire. There are other reasons to use the passive but this will help to understand why we use it.

Passive voice is mostly used in academic writing. Your second example would sound weird in daily conversations. You should not write "I'll tell him." in your academic assignments, presentations etc. as well.