To write a good birthday message for your son, write how you feel about him and build his self-esteem by saying how proud he makes you. Celebrate his achievements while telling him you are excited for his future.
Appropriate birthday messages vary depending on your son's age. Young boys are usually focused on their presents and eating cake, so a short message works well. Preteens and teenagers may not want a sentimental or emotional message, in which case a short, light-hearted message may be best. Adults are typically open to any type of message.
Sample birthday card messages for sons are available at the Best Card Messages, BirthdayMessages.net and Wishes Messages Sayings websites, as of March 2015. Use these examples to give yourself an idea of what to write, or pick one that captures how you are feeling and use it to start your message.
You can write about memories you have with your son, things he is good at or what you admire about him. You may also want to tell him that he is the best part of your life. A good joke works well to lighten the mood, especially if the rest of the card is sentimental. A birthday message is a way to express your feelings more than usual.