His own digital watch is an even better idea. (My DD would never agree to wear a digital watch--she doesn't like the way they look.)

I've also considered getting a visual timer, like these:

http://www.amazon.com/Time-Timer-LLC-8%2522-Inch/dp/B000J5OFW0/ref=sr_1_1?ie=UTF8&qid=1363120392&sr=8-1&keywords=visual+timer

http://www.amazon.com/Learning-Reso...=1363120392&sr=8-6&keywords=visual+timer