I think I'd respond. But just with something like:
Thanks. You too. Sweet.
This recognises that he's made the effort to acknowledge your special day, but keeps him in the category of 'friend/co-worker' - i.e. what you'd say to an acquaintance who wished you well.
To me it seems like a pretty big thing that he actually sent you the email. My stbx has given up acknowledging my bday or anniversaries. Reward the good stuff but don't pressure him.
