Gosh, a dilemma. You could send him an e-mail happy birthday or card through the mail. It's kind of just keeping the lines of communication open. On the other hand, if it is just too "wrong" right now, it won't be the end of the relationship if you don't. I think it's your call.
I think that you realize that, even though you're mad as heck at him, you really would like to see him find a good recovery. And in order to send that message to him, you have to be in at least minimal contact. That's what many of us here on SR are doing - keeping just enough contact to keep the relationship going without taking a front-row seat to their addiction.
Hope that helps.