I think the bottom lie of all of this is you need to decide whether you are dating a grown-up or being a mother to a kid, and I don't mean that to sound as prickish as it's probably coming off. I sense you might be flipping back and forth and if I'm right, I can almost guarantee it won't work out for either of you.
I went through a little of that early on when I met my VYW in 2010 - she turned 20 just before we met, and I was in my 40's, a single father, a business owner, totally in a different life-place. It took me a few rounds of accidental fathering to get it and pay attention to it. I'm glad I did because here we are in 2016 and we're still going strong.
I didnít get to finish the update because I had to go to church. So I guess there is nothing wrong with Derrick promising that he would be open about his smoking and then breaking that promise when we were with his family on Easter? Itís not as if most of the adults in his family donít smoke (parents, grandmother, aunts). He wasted a good opportunity to get this behind him. And something hidden always gets bigger and worse the longer it is hidden. Because things always come out eventually.
On another note, Derrick reconnected with his parents at parents during Easter and now they are talking to him and theyíre talking to me. This is wonderful! But itís not going to be so wonderful a year from now when they find out their son is smoking, because theyíre going to blame me.
What does hiding solve? If youíre hiding something, youíre still doing it.
What Trish said... you can't parent him. Someone else did that already. They may not have done the job you would approve of, and when that is the case, then you have to think about what you want in your life. This is who he is. Do you want that in your life? Can you live with it, without blaming him and nagging to change constantly? If the answer is yes, then its yes, stop punishing him for something you are willing to accept. If the answer is no, then move on. You can't parent him, but you can decide on the course of your relationship with him.
Psycho hatchet wielding midgets deserve to die
Why does he need to tell them he's smoking? How is this going to "get bigger and worse"? I genuinely do not understand what consequences you are fearing, as far as his family goes. I don't know why you think they would blame you. "Teenager Takes Up Smoking" is not very shocking. And if they did blame you, so what? What do you think the consequences of that would be?
Also want to +1 the example of a 48 year old. Would you be making a 48 year old promise to tell his parents he's smoking?
I really think you need to evaluate your ability to treat this person as an adult.
I think you're missing the point here, which is not about whether it is right for Derrick to hide his smoking or even about him saying he would tell them and then not. It's about you telling him what to do as if he were a child. You're having a sexual relationship with him. You cannot treat him like a child!!
I do have to agree with many of the others here. It does seem you are treating him as though he's a child and you are the parental figure. I just don't feel that's healthy for a relationship. Even giving him cigarettes seems as though are you putting yourself in the control seat and either approving or disapproving of his behaviors and acting as though that's your role.
Just because he's so much younger doesn't mean he's not your partner. I expect my husband to act like the adult he is and treat him as though he's an equal in our relationship with equal "say" and working on equal responsibilities. I tend to take more on myself but am trying to get better about that. It's not because he's so young though, feeling the need to take care of everyone else has been a personal issue of my own for a long while.