I'm not familiar with this. This might be a stupid question, but what is the letter to his mother and where is it found? Truthfulness or non-truthfulness of dreams is an intriguing topic to which I haven't given much thought; I'd like to follow this up.
If I'm not mistaken, its found within the Epicurean Inscription commissioned by Diogenes of Oenoanda. Vatican Saying 24 also gives some insight into this as well.