where to begin...
1) smoking is a personal choice and an addiction. you cannot MAKE him quit. he has to want to for himself. you can support him regardless of his decision if you truly love him.
2) if you are worried about your child you have the right to explain to your fiance that if he chooses to smoke he will have to abide by some basic things in order to be with you and your son. he needs to smoke outside (no matter what the weather). he cannot smoke in any vehicle that your son is in. it may even be prudent to have an old coat or shirt for him to wear while smoking to help keep the smell down to a minimum. and he MUST wash his hands when he is finished smoking, and before touching your son. (all of these helps prevent a smoker from passing on second hand smoke to a child. i would also advise you to request that he not leave his cigarettes and lighter (matches,etc) anywhere that your son may be able to get them.
3) If your fiance truly loves you and your child, he should not be upset by any of these requests. My husband smokes and does not complain one bit about these very same demands upon him. Our oldest child is asthmatic and so we have to be extra careful around her.
4)if he truly wants to quit for himself then, you can only support him. he can't, and likely won't, quit for you or for your little one. he started smoking for himself due to whatever was going on in his life at the time, you cannot change this with ultimatums and guilt trips. I would love my husband to stop smoking, but until he wants to, I make sure he follows the rules regarding the kids and, because i love him, i deal with it.
5) regarding the fact that he may have started and is hiding it...it may have to do with the fact that if he is, he is not ready to admit to even himself that quitting this time around has failed. it could be that he does not want to have a fight with you about the subject, or that because you want him to quit so much, he is worried you will leave him if he has started again.
6) if he is smoking again you can tell when he kisses you. even if he has recent brushed his teeth, you will be able to taste it during the kiss. give him an out (an easy way to tell you) and ask if he was around someone who was smoking that day. See if he confesses it was him or tries to change the subject.
7) whether he is ready to admit it or not (or if he has quit and it is a non issue) tell him, if he is struggling with it and having one now and then to try and get through the hardest parts that you support his effort but to please make sure and abide by the "rules" in number two for the health and safety of your son.
If this came across as preachy, i apologize. I myself never imagined i would marry a smoker. But, I absolutely adore my husband, and as long as he keeps our children's health safety foremost to his bad habit, then he can continue until he is ready to stop.
Good Luck... ;-)