Well, smoking isn't good for you (and all the other clichées
) no matter what. So weighing him smoking against the bad things that can happen if he doesn't is not a valid reason as to why not support it. Which you already know, sorry.
The problem in this case (IMO) isn't the smoking in itself, but the "safety" he gets out of it. It's a habit of his, and when you have to breake a habit...well, we all know what that's like.
Try to suggest other things he can do on the occasons when he used to smoke (replace a habit with another), and support him, no matter what he's like (if it's because of withdraw).
Talk to him on a daily basis, preferably at the occasions when he usued to smoke a cigaret, (if he had any specifics, like after breakfast, at 10 o'clock, etc) about the benefits about quitting smoking (to make him more aware of them and for motivation), and make sure he knows that you understand why he's irritated for no appearant reason and does things he normally wouldnt.
Understanding and enabeling is the best way to control the situation.
I hope everything turns out well for you both- good luck!