Tomorrow is my ex-boyfriend's birthday. I want to send him a message. What if I send him a happy birthday and he just says thank you?

Send it to him if you want, but in general, you shouldn't just say thank you, even if he only replies. You don't have to care too much. It seems that you still care about him in your heart. If you still want to be with him, just say so and give him a chance. If it is just a greeting, it is normal for him to say thank you.