T K Maxx-my fav shop in the world- has stuff like quiksilver hats or wallets for your budget. You can't order online but they do have a website to find your nearest store
I would think any small gadget would be acceptable.
How about a pedometer which he can clip onto his waistband to tell him how many paces he's walked every day? You can probably get one for under �10 from a sports shop, or try Googling Pedometers and you'll probably get lots of websites to buy them online.