There is no real specific way to do it.
You could write, if his name is for instance François :
"Mon Cher François", "Cher François", "Très Cher François"
or more specific "Mon Cher Petit François" (Dear little ...)
The French title, Mademoiselle means my young lady and is the proper way to address a young lady in French.
A... Toddler Kid Child
a young lady is called 'une demoiselle' in French. You address this unmarried woman as 'mademoiselle'
Call her 'Madame', or 'Mademoiselle' if she looks young and unmarried.
Mademoiselle is French for "miss", the formal address for an unmarried young woman.
Traditionally, in French a young or unmarried lady is referred to as "mademoiselle". A woman or married lady is referred to as "Madame".
Tot .
The word "boy" in French is translated as "garçon." It refers to a young male child or teenager.
A babygirl is either a young female child, or a friendly or intimate term of address for a woman.
kid. the child of a goat
It is difficult to answer in a specific way because all depends on the content of the letter or purpose and who the letter is address to , like a family member, friend and so on. But to answer that would be, " Tu amigo, ________ . Meaning " Your friend_______.
Misses or Mlles