"who am I to deny him those?"

Well, you're the cook, aren't you?
Seriously, you don't make anyone eat. They choose what they eat, just as you do. I'm sure he's thinking "But I've always eaten this way" and can't realize what it's doing to his body, but you can't mess up your health just because he seems to be messing up his.
If Atkins works for you, then keep with it, and just quietly let the results speak for you. Being honest, as you have been doing, about how sugar and bread makes YOU feel is all the reason anyone should need to understand why you eat this way.