My teen claims he's only smoked once or twice, will he be predisposed to addiction for the rest of his life?

Good question: Using any addictive substance in the teen years does increase addction risk. Teens brains are not fully developed in higher level functioning such as desicion making. This is why they take more risk in the first place. Obviously many teens do risky things and not all become addicted but yes the risk is higher. Full addiction also impairs this normal development further.

Not necessarily: Smoked cigarettes? Cannabis? Not smoking cigarettes or using tobacco of any kind is clearly the way to go. Some kids will try it and put it down. On the other hand, those who are lifelong smokers usually start when they were teens. I think you are right to try and get in between your kid and smoking. Good for you. Keep it up. Do not allow it. Do whatever it takes. Parents can give up. Don't.