You just flip somebody told him into he were told by somebody. So the original:
- It’s high time (that) somebody told him the truth.
becomes this if promoting the indirect object to subject:
- It’s high time (that) he were told the truth by somebody.
or this if promoting the direct object to subject:
- It’s high time (that) the truth were told to him by somebody.
It would be more common to promote the indirect object as in (2) than the direct object as in (3).
In formal writing, it would be were not was in the passive version. That’s because the verb following it’s (high) time (that) has historically been in what was once called the past subjunctive in earlier forms of English, and which is retained morphologically only with be. Casual conversational language and writing in a less-than-formal register would more often use was, though, especially in the UK rather than the US.
Occasionally these it’s time constructions take for–to infinitives instead of that clauses:
- It’s high time for somebody to tell him the truth.
would become
- It’s high time for him to be told the truth by somebody.
or, at least in theory, but it sounds pretty bad:
- It’s high time for the truth to be told to him by somebody.
In comments to the original question, the asker wondered what an equivalent simple sentence might be so that we don’t have two different clauses to consider. I had this to offer:
For comparable simple sentences instead of complex ones, with minimal meaning change, I would use the strongest deontic modal of obligation available (ᴍᴜsᴛ) and then add something like finally/belatedly/at last/at long last to incorporate the ‘high time’ notion that this truth-telling is overdue. So perhaps “Someone must finally tell him the truth” paired with “He must finally be told the truth” will work for your non-complex versions, although I cannot recommend “The truth must finally be told to him” for most (if perhaps not quite ᴀʟʟ conceivable) circumstances.