To compose such a letter, you need to pay close attention to the letter-writing conventions of the period. Chartier's book (listed below) is one of the best concerning the letter-writing manuals of the period. Even personal correspondence was highly formulaic, and an educated nobleman would have been well-versed in the appropriate conventions. Consider what form of salutation and complimentary close would have been acceptable given the circumstances. Would he have used formal convention of address and date?
First, in terms of audience, this would be a letter from a noble to a provincial bourgeoise, in other words from superior to inferior. How would this social relationship colour the letter?
For register, what types of diction, politeness, and modes of address are appropriate? Would Boulanger's sense of position preclude extreme informality? How would their intimacy have affected appropriate language?