I feel for you. My son is only 2, and the repetition is already very frustrating. Unfortunately, repeating yourself, calmly yet firmly, is the best way to get through to your kids.
While the temptation to resort to yelling is hard to resist (and you're not alone! This article references studies that show that nearly all parents do so at some point), I believe it sends a message to children that it is okay to yell back. The fact that your son yells right back at you would seem to support this, and I know other parents who have experienced the same.
If you find yourself losing your calm, take a moment or two to remove yourself from the situation, gather yourself, and then come back.
This question, while describing a situation more extreme than yours, does have some useful advice in the answers. The key takeaway for me from one of the answers is: "Children key off their parents' reactions more than the argument or physical discipline itself." I think this is fantastic advice, but it can be very difficult to change how you respond, especially with a defiant child.
Losing privileges is probably an appropriate response if he continues to engage in poor behavior. If he pushes or hits his brother during a commercial, the TV gets turned off (or perhaps his brother gets to change the channel to a show he wants to watch, instead, or even his brother can stay and watch TV, but he has to go to a different room).
Asking a 5 year old to sit down and relax may be expecting too much, though. Especially if he's been diagnosed with ADHD. Instead, try to find ways for him to burn off some of that extra energy. Trips to the playground, games inside or outside of the house, or even a walk around the neighborhood are better alternatives.
I also absolutely agree with Vicky.
If you disagree with the medications prescribed for your son, find a different doctor. Don't just pull him off the medications and devise your own treatment plan. ADHD is arguably over-diagnosed, and I don't believe that medication is always the right course, but that's not a decision to make lightly, and you should have professional guidance in whatever course you follow. Especially if your son is having significant behavior problems.