There is no one answer to this question as it depends on what style of love poem you wish to write, and what your relationship with the person is. However, here are some general tips on how to write a love poem for him:
Think about what you love about him - what are his best qualities? What makes him special to you?
Use imagery and descriptive language to paint a picture of your love for him.
Be personal - this is your poem, so make it as unique to your relationship as possible.
Use your emotions to fuel your words - write from the heart.
Don't worry about being perfect - just let the words flow and capture your feelings for him.