To be perfectly honest, you lied on your entrance interview. It's probably best not to talk to your employer about this. What would you have to win? Just keep your head down, try to make sure that you aren't smoking in times or places that would let your coworkers catch you (either by seeing you or by smelling it on you) and drive on. If that's overly onerous for you, I suggest you look for another job at the same time.
This may vary somewhat by location - I suspect there are some places where you are protected and some where you are not - but you have a company of 25 people, and this is clearly something that the CEO cares about. If the CEO knows that you smoke, he will be hostile to you (both for hitting one of his hot buttons and for lying about it), and in a company like that, having the personal animosity of the CEO is the sort of thing that's guaranteed to make your workplace experience significantly worse.
Also, a lot of places are at-will employment, which mean that you can be fired at any time for any reason. Even if the real reason is "you're a smoker" and that is protected, I guarantee he can come up with some other reason (or no reason at all) to fire you that won't be worth your time to litigate... especially with the difficulty of getting a judge/jury to sympathize with a smoker's rights issue.
There is no win here.