I have 6 years experience with 2 adhd stepkids now 20 and 15. My advice to you is stepmom related not adhd focused: work directly with your husband and let your husband primarily handle this. It’s his son. Understand that this is not a one time fix, it’s on ongoing process and a project. Talk with your husband and come up with an action plan together. Do not lead. Let your husband lead, not because he’s the man (obviously) but because he’s the child’s father. And yes in our case too the birth mother is not helpful and is counter productive, most everything we accomplish on our week is undone in her week. That’s classic and is simply one of the challenges of blended family. However I sense you are a competent get it done woman. Just be careful and don’t burn yourself out. Your top priorities are taking care of yourself, your own son, your new marriage and of course being a good stepmom but you’ll be fine there because you’ve already demonstrated you are someone who cares. Believe me, if you jump in and take this on it’s very hard to back off later. Make sure you ask your husband for at least equal involvement on his part. It is after all his child and his responsibility.