Actually peeing on the floor and not doing as told has everything to do with ADHD.
Children with ADHD have daytime enuresis (wetting accidents) rates three times higher than children without ADHD. And nightime rates are five times higher. (p. 72, The ADD/ ADHD Answer book," by Susan Ashley.) And, kids with ADHD act with no "filters". They don't take the time to think about what they have been told - they just do.
Now having said that, by age 9, the enuresis should be getting better.
Obviously if he has no problem when he is on medication, then the fact he has a problem when he is off medication would indicate where part of the problem lies. Typically, when off meds, the kids can hyper focus and ignore the peeing signals until its too late - and then they pee. I am wondering if this spot is close to the tv or a video game?
You have some choices here.
Keep him off meds. Note when he is most likely to pee. Make sure that he goes to the bathroom before that time. If he doesn't go then, make sure he tries again in 30 min. and repeat.
The other choice is to keep the meds going all the time. Lets face it, you don't like it when he is off the meds. By the way, you really need to understand why he is acting the way he does when he is off the meds. If you can this will help you deal with him better. And being "very smug and disobedient" is kind of typical for a 9 year old starting to feel mature. The ADHD just really makes it worse cause they don't have the filter to say - oh, oh - I should not have or should not do that.
Taking him off meds for the weekend so he can eat is not that good of an idea. He really can't eat enough on a weekend to make up for a week of not eating. For that matter, from my years in elementary school, I have seen a lot of boys who are so intent on getting to the playgrounds that they just don't stop to eat. So it might not be the meds, but it is a typical side effect. I don't know how long he has been on meds. But many children do adapt to the meds and the side effects disappear. (P. 156, The ADD/ ADHD Answer book," by Susan Ashley.)
What he does need is a very good breakfast every day. Also it is very important that his meds are tuned so that they are effective when they need to be. By dinner time, they should be wearing off so that he can eat a good dinner. As he gets older, then the need to study at night will change that schedule.
There are a lot of resources out there to help you. For example, this site has a lot of great food tips - http://www.additudemag.com/adhd/article/6102.html
And I really think that it would be worth the 10 or so bucks to get Ashleys book here - http://www.amazon.com/The-ADD-ADHD-Answer-Book/dp/140220549X
Finally, if I can be of any more help or if you need more resources please feel free to post to me on the ADHD forum where I am also the CL.
http://www.medhelp.org/forums/ADD---ADHD/show/175
Best wishes.