Hi Toni-
Here are a few simple ideas that I did in my son's previous room:
1) Purchase already cut out round pieces of wood at Lowe's or Home Depot. They are already sanded and finished. Paint them to look like basketballs and have them 'bounce' across the wall. You can paint 'bounce' marks on the wall with black paint or just leave the wall as is. In order to attach them to the wall I attached picture hooks from Home Depot and nailed them to the back. I am always on a tight budget and this was very cheap.
2) Keep your eye out for a free basketball hoop on Craigslist or even a cheap one at the thrift store. You can either keep it attached to the backboard or reattatch it to a larger piece of plywood, so he can actually bounce a real ball off the wall without damaging the actual wall. The larger piece of plywood can be painted to look like a crowd, scoreboard, etc.
3) Hang old jerseys on the wall or enlarge photos of your son playing basketball-action shots would be better-but your 'typical' sports picture would do as well.
4) Cut out letters to the name of his favorite team and string it onto a really neat rope or string and have it go from one corner to the next.
Get him involved. You might be surprised at how interested he might be. My nephew just moved here and painted his own wood valance and has completely decorated his own room. My only suggestion is to go neutral and to not purchase a lot of 'premade' sports items. Those might be fine for a younger child, but might not be very 'cool' for a teenager. I would also stay away from specific team colors, as their likes and dislikes change so often. Let me know if you need any help with anything, I would be more than happy to give you more suggestions. Good luck!