Brothers break their arms. They just do. This means that siblings are obligated to comfort, help out and mercilessly rib them for it until the end of time. It is the way of the world. Gifts for such an occasion can range from practical to hilarious and all are appreciated for exactly what they are: tokens of love -- sometimes with a hint of mockery.
Being a loving sibling means never letting him forget. You could get him a personalized cell phone cover with the photo of his x-ray on it. A T-shirt that says, "I Do All My Own Stunts" is a fun gift, too. You could also hit up your local trophy shop and have something made for him that reflects the story of how it happened. Any of these gifts will commemorate the incident, delight your brother -- and potentially justify payback somewhere down the road.
A broken arm can mean time off of work or school, so providing you brother with some entertainment and favorite treats will stave off the boredom. Include books, magazines, crossword puzzles, a deck of cards, candy, chips, those tiny liquor bottles he can open with his teeth, and whatever else you know he would like. Then, if you are feeling particularly cruel, you can add a 2000-piece puzzle of a blue sky. That should keep him occupied.
If you actually want to be helpful, give him something he can use. A waterproof cast cover is an excellent start since showering can be a real bummer with a cast. A sling for comfort is good, too, as it will keep the arm immobile, reducing soreness and swelling and also serve as a reminder to be more careful. But maybe the best thing to get him is a cast scratcher. It slides right between the skin and cast and provides the kind of relief that will keep him sane.
Give Him a Hand
Literally, give him a hand. Help him out with chores that are difficult to do with just one arm. Since he still has to eat, you could cook him dinner and leave the leftovers. If you're not so handy in the kitchen order him some take-out. If all you have to offer him is time, do that. Distract him. Play cards, beat him at his favorite board game, watch that stupid movie he really likes, or do whatever it is that will make his recovery a little easier.