It is common to feel abandoned when you are left alone.
Yes, you can say that! Or you can say “letter that contains advice” or “letter that provides advice.”
By referring to them as "his mother and his father" the reader can feel more close to the character. When saying "the mother and the father" one can relate less, feel distance between one and those parents. On the other hand, when saying "his mother and his father" the reader feels a little more close to the situation, feels deeper the relationship and can ever reflect him or herself in the character.
