Two examples of letters to grandsons are from The Grammar Curmudgeon and from Family Friendly Poems. They offer two letters that both offer advice but in two different ways.
The Grammar Curmudgeon has a letter that is read by the grandson after the grandparent's passing. It is detailed and urges the grandson to take advantage of his youth and appreciate the humor of life.
Family Friendly Poems has a letter from a grandmother to a grandson she has not met. She assures him that he has great things in store for his future and that he has a wonderful mother. She tells him she loves him even though they have yet to meet.