This is great advice!
I have ADHD, and so does my son. My husband has to deal with our quirks on a daily basis. I know it wears him out. It wears me out, too. We argue often about how to parent our oldest son. He is stern and strict, while I am more lenient and sympathetic.
It may be helpful to watch some parenting videos or read articles on the ADDitude website together to get ideas. Let your wife know that you care about her son, and you want to be understanding of how to best help him while getting him to be a responsible teenager. I think setting out a plan for the week is a great idea, letting him know what he will be responsible for. TBRI is a great parenting strategy if you can get your hands on some of the training videos. Regardless of how you do it, you and your wife have to open up lines of communication about parenting together. Being a step-parent is no easy feat, but you seem like a smart man who has been reading up on how to interject yourself appropriately. I think it’s amazing that you care as much as you do! Your wife should be grateful to have a husband who cares about her son and that you would take the time to reach out on here. Overall, you’re thinking on the right lines. He needs to be responsible and held accountable. He CAN DO IT. My son and I both go to therapy in addition to taking medication. It’s a balance. I hope that your wife can see that you care and love them, and that you only want the best for your stepson. Good luck!