"I had hoped watching their father struggle to quit would have put them of smoking"
Unfortunately, it doesn't work that way. Teens whose parents were are smokers are 10 times more likely to themselves become regular smokers at an early age or to experiment early on with cigarettes than kids with nonsmoking parents.
http://www.reuters.com/article/2014/05/13/us-teen-smoking-parents-idUSKBN0DT1QV20140513
My parents were both smokers and all of my siblings became smokers at an early age. My parents used to blame peer pressure, but it was their own example that facilitated experimentation and addiction.
Has your husband been prescribed any medication to help him quit?
If I was you, I'd ask my son if he has been smoking. Maybe he has been around friends who are smoking. I'd start with an honest conversation.