I know what you're going through
First the bad news and you probably don't want to hear this - but your doctor is 100% right about the smoking verses weight gain.
Dr. Mercola has something to say about this.
Ditto for the Zyban/Wellbutrin (my husband got suicidal on Zyban)
Smoking is better for your DH then gaining back his weight.
Now to the good news.
It is possible to quit smoking. I know because I used to smoke. I haven't had a cigarette in almost 30 years.
Every time your hubby quits the odds are greater that he will stay off them for good. He sounds like he has the desire to quit (unlike my DH) and that's half the battle.
I don't have a perfect marriage - but I do have a strong one - over 22 years.
What I do in my marriage is pay his life insurance premium without fail, and I don't allow my husband to smoke in the house or in my presence.
He knows how strongly I feel about this issue, and I have told him that if he smokes in the house or in my presence I'm moving out.
And I mean it.
For me, his smoking is about a lack of respect for my feelings and fears. He knows that I can't change him or what he does, but I'll be damned if he'll throw the smoking issue in my face! I also almost never kiss him - very sad
But the fact is,
he has picked smoking over me and that was a hard thing for me to admit to myself.
Smoking is just like any other addiction (mine is food). It is possible to control addictions. And unlike a food addiction, a smoker NEVER has to smoke again - never. For us food addicts we walk the tiger at every meal.
Your husband can do it if he wants too bad enough. And you can find a way to live with him happily in spite of it.
Good luck.