How long has it been?

People don't typically move on within a week or two. If they do, they were by far not worth the effort.

It seems like he cares because he wasn't happy with the break up. And he took a risk to share something important with you - which in turn, you had a bad reaction to. People don't take risks if they're not willing to invest in the relationship.

So while none of us can know for sure whether or not he's moved on, you will also never know unless you contact him.

Sending a happy birthday message is quite harmless. You've picked out the worst case scenario for his response. He may give a polite response, he may engage in a conversation, he may simply not respond. Those are three other possibilities.

And it will help you know one way or the other.