Somebody promised to tell him the truth
Could potentially become a passive expression as:
He was promised to be told the truth
I say "could" because the first sentence doesn't indicate that the person who is supposed to be told the truth is also the one promised the truth. For example:
(1) Bob promised Alice to tell Charlie the truth
compared to
(2) Bob promised Charlie to tell him the truth
Either of those sentences would match "Someone promised to tell him the truth"
The passive of (1) would be
Alice was promised (by Bob) that Charlie would be told the truth
The passive of (2) would be
(a) Charlie was promised (by Bob) to be told the truth
or
(b) Charlie was promised (by Bob) that he would be told the truth
While (a) is possible, it's more likely that something like (b) would be used. Note also that the active versions, the person promising must be the same as the person telling the truth, but in the passive versions the person telling the truth could be different from the person promising (but the person being promised is also the person hearing the truth)
He was promised to be told the truth
This is a sentence which could be understood, but you are far more likely to see the active version.