1. If you are interested in him or like him:
"Wow, you are very poetic! I like this poem very much and think you are very romantic. "
2. If you are just friends or unfamiliar:
"Oh, thank you for sharing. I have read this poem and it is well written. "
3. If you don't want to continue to communicate with him:
"Well, all right. Thank you for your message. " (relatively cold reply)
In short, the way to reply depends on your feelings and relationship with him.