Under the advice for living with someone with ADD is listed "avoid caretaker/sick person." What I continually see here and in other places is the non-ADD partner (usually woman) feels like she's the mother in the relationship. I just hate that. My guy's on meds and is in therapy and things are going pretty well. I mostly don't say do this or that. He can only bring his dishes to the counter, but I just leave them there until he gets around to cleaning up. In almost every area, he will finally do something -- clean out his car, pay his bills, file stuff, call his kids, etc. But I have one giant hole of resentment -- money. I have it because I've saved (and could work regularly); he has almost none, because the future never occured to him. We're about to retire and I guess I'll be almost totally supporting him. We're looking at a big purchase; I told him how much I felt I could spend without compromising my savings. A couple of times a week, he'll come up, having found just the thing to buy, that's always way more than what I said. I get angry -- "don't you understand that this is all the money we have to live on and I"m not going to be reckless with it"? I feel like a shrew, he puts me in the position of being the bad guy or the mother, having to say no to him. I guess the whole area is something we need to talk out; I haven't yet been to his therapist with him, but I want to soon. Any advice?

Get to that therapy session!!!!! RUN TO IT if you have to. And be consistant about it. Depending on how long he has been in therapy he may have worked though his own issues and is ready to confront his relationship issues. He needs you there to do that. If you are willing and consistent in attending therapy sessions, things should begin to improve.
Giving him gentle reminders to do things around the house or pay his bills, can get frustrating and I see why you would feel MOTHERLY to him, but we all need gentle reminders sometimes, we just all need them for other things. Instead of, "PUT YOUR DISHES IN THE SINK, I AM NOT YOUR MOTHER, AND I SHOULDNT HAVE TO CLEAN UP AFTER YOU" or instead of leaving them there, Maybe you could try, "Those dishes gunna hop in the sink on their own?" AND THEN LAUGH AND MEAN IT!!! It will give you both something to laugh about, and will tend to cut the tension :)
In regards to your large purchase, I do see it fitting to ask....why is HE picking it out if YOU are the one paying for it? How about if it is something your both going to be sharing, you shop for it together, and gently remind him when he sees somthing that is over budget! It has been MY experience that persons with ADHD (men generally) tend to see what they want and just GO FOR IT...taking NOTHING like price, safety, durrability etc. Into consideration...Could it be he is just anxious to make the purchase of his dreams? Either way, duke it out in therapy, tell him in therapy that you are not his mother....make it known your his partner, but yeah...get to that session, and my guess is it will take A LOT more than just ONE therapy session for everything to become hunky dorry. Good luck with this one :)