Poet Sophie Jewett's "To A Child," Langston Hughes' "Mother to Son" and Rudyard Kipling's "If" are all good choices for a poem to a son on his birthday. Each poem celebrates the relationship between a parent and child.
"To a Child" by Sophie Jewett is a poem about a mother's need to protect her child. She mentions that she had a dream that she was a tree, and her child was a bird she sheltered in her branches during a storm. When the storm passed, the bird filled the air with song. This is a good analogy for the way a mother protects a child and a child brings joy to a mother.
"Mother to Son" by Langston Hughes features a mother who tells her son that her life has been difficult but that she never gave up. She encourages her son to follow her example and persevere through life's challenges. This poem would be appropriate for a son who is facing difficulties and needs encouragement.
"If" by Rudyard Kipling gives a son a series of advice, such as trusting in oneself even when others don't and continuing to take risks even when failures occur. He encourages patience and virtue, and in the end, he tells his son that if he can accomplish everything set forth in the poem, he'll be a man.