Typical dilemma in step-parenting. I'm a stepmother and I can relate to the differences in a "weekend" child - dads want to be Good Time Daddy and not make it such a rigid environment that the child never wants to come. If your husband has guilt about not spending enough time with his son, he needs to address that by spending more time. If he can't because of a rigid visitation schedule, then he needs to get rid of the guilt because it's not helping and it's beyond his control.
ADD meds are a whole issue unto themselves. They cause side effects and it's frustrating when kids are on them as well as when they go off them.
Sometimes they cause personality changes which are uncomfortable for the person taking them even if they make life more pleasant, at least at first, for others. So what's the best choice? Hard to say.
The real problem here is that you and your husband do not agree. It's made worse by the fact that he and his ex don't agree and don't compromise. So the son has no idea what's coming next, and he has no idea how to behave or control himself, even if he wants to. So of course he acts out to get attention. And he's getting it.
The answer may be, in part, for you and your husband to come to some understanding (and maybe you need some help from a counselor) because this is affecting your marriage.
You also are trying to protect your daughter and your furniture, and maybe that's not giving this boy enough flexibility when he comes into a new environment on a weekend or for a 1 week summer vacation. He can sense your irritation and disapproval. Some kids also play off the different opinions of their parent and step-parent - if you and your husband allow this, you're making the problem worse.
And your husband has to step up and be a father, not just a weekend party planner.
Go ahead and set up something separate for your daughter to do so that your husband and his son (and maybe you) can do something together. Most 15 and 12 year olds don't get along perfectly anyway, and boys can be later to mature in many cases.
If you can find a water park or a movie that all would enjoy, great. If your daughter can have a sleepover at another friend's house, fine.
But the biggest problem is that you and your husband are so far apart on this issue. I'd start there.