Play board games that can be managed with one hand . Set up an ebay account and see if he can increase pocket money by selling unwanted goods online . Learn to use a camera with one hand & learn photography ... Enter competions to see if he can win a prize for when he is better - I know its not as good as swimming or bike riding ... I had a month of no swimming after an accident so know how annoying it is .
There are plenty of things he can do like fishing and when my son broke his arm he still played on the slip and slide, but he can go hiking he can also play games on the computer by using just the mouse and his other hand but he can be creative on his own and think of things he can do.
Coffee filter art: you can take eye droppers and fill them up with food coloring and create your own master piece; create a book filled with his and your own story accompanied with pictures he drew or painted.
Simple arts and crafts like this are easy to handle, even with a broken wrist. Audio books are good. I've seen many children at this age handle difficult tasks that you would think impossible. Hang in there, he'll be back to normal in little time.
I say spill soapy water in a cookie sheet/ thin pan and let him blow huge bubbles with it by using a straw and blowing through it into the pan of soapy water. Have a contest to see who can blow the biggest one. Another idea is playing homemade lawn darts. You put a golf ball into a piece or sturdy fabric and then rubberband it tightly. Toss the "dart" into hula hoops spaced around your yard. You can even make this a golf course type of game by having different sized hoops or items to toss the dart into.