Hi there. I'm Alicia, I'm happy to help you today.
To be quite honest with you, I don't think there's anything wrong with sending a text message to wish him a happy birthday. It doesn't have to be long or complicated, and unless things ended badly between the two of you or there's other reasons you might have for not sending the message (and if you want to talk about that, just reply to this message here), he might actually appreciate hearing from you. There's nothing wrong with staying in touch with an ex, especially if you still care about him. Please let me know if you want to discuss this some more.