A good birthday message for a teenage boy is "Today is a great day because you were born on this day! Get out there and have a super awesome birthday!" Make a birthday message to a teenage boy playful and fun while flattering him at the same time.
Another effective technique that appeals to a teenage boy is adding a joke to the message. Most teenage boys enjoy humor. If the boy receiving the birthday message has specific interests, incorporating those interests is also effective. For example, if the recipient is interested in hockey, use "Lace up your skates and get going! It's your birthday so go out and score some fun today!"