Congratulations on kicking the habit!
Your best bet is to book a cruise 6 months to a year in advance, as you have a better choice of cabins and are usually eligible for the early booking discounts. Sometimes you can get good last-minute deals on cruises, but they are often whatever cabins are left over, and you also have to consider whether you can get to the port last-minute (air seat availability & cost). Some cruise lines will allow your travel agent to get you a lower price if they have subsequent pricing specials after you have booked & paid deposit, but they will probably not volunteer the new price info - you or your agent will have to keep up on it! Other cruise lines (Carnival among them) will not allow that, but will make you cancel and rebook to get a lower price - and you take your chances that as good a cabin is still available.
If you are considering a cruise or a cruise line that offers only a few sailings (exotic ports, or one of the short cruises from East Coast cities, for example) then you should be sure to book way ahead (9 mos to a year). The same is true for any of the small-ship lines like Cruise West or ACCL, as their capacity is limited to start with. If you are instead favoring a big-ship line in the Caribbean, then 4-6 months ahead should be ok, especially if you are not fussy about the cabin you get or can be flexible on the sailing dates.
If you think it will be extra motivation for your husband, you might consider Carnival's Paradise - the only completely nonsmoking cruise ship afloat! They are very serious about it - anyone, whether passenger or crew, who is caught with smoking materials on board is put off at the next port and must find their own way home at their own expense! Paradise offers 7-day Caribbean cruises year-round. Other ships allow smoking, but almost all restrict it to certain areas; dining rooms and show lounges (during shows) are smokefree, for example.
If you don't sail on Paradise, you may want to limit your time in the bars and casino onboard, as the majority of the smoke is there, and he might be too tempted. You may also want to avoid ships that offer "cigar bars" for the same reason. Depends on how strong your hubby stays! Good luck to both of yoU!