One way to write a poem for a friend is to review a mental timeline of tour friendship and write about a special or poignant moment from that time period. This may include when you first became friends or the first clear memory you have of him. This approach provides specific evidence to your friend of the major influence he has on your life.
Another way to write a poem for a friend is to concentrate on your favorite memory of him and write about it. Details may include your location and your thoughts at the time, why the memory stands out to you, and why this memory is part of your reasons for considering your friend someone you have chosen as a family member.
Conversely, you can write a poem about a time your friendship was very difficult or strained. Describe the resolution of this personal conflict and the impact this struggle with a friend had on you.
Make a list of your friend's personality traits, and base a poem around them. Highlight any times his personality has changed over the years and how it interacts with your own personality.
Regardless of the method, share the finished product with your friend so he knows how much you value the friendship that you both have.