The passive voice of “He will do it” is “It will be done by him“.
When you transform a sentence from active into passive voice:
1.The object of the active voice sentence ( in our case It) becomes the subject of the passive voice sentence.
2.The verb consists of a form of the verb – to be in the corresponding tense ( in our case future simple- will be ) + the past participle of the verb in the active voice sentence ( in our case – done).
If we want to mention who performs the action, we use the by phrase (in our case by him.)
You can read more about passive voice and do a quiz here