Lose the guilt. It can rule your life and alcoholics are notorious for laying it on us without us even knowing it.
You know what you should do for his birthday? Go out and buy yourself THE BEST birthday present ever! Spend the money you would spend on him (and I know it would be a relatively large amount) on YOU! And while you're buying it, you can say out loud, "Happy birthday Bob." and it will meet your criteria that he not know
Treat yourself just as good as, or better than, how you would treat him. You deserve that.