Lots of sweat glands there. The testicles to make sperm need to be a degree or 2 below normal body temperature. That's why they "hang" outside the body. (In cold weather they retract).
So to keep cool they hang, and there's lots of sweat glands. Given the underwear and pants we have on, that sweat often takes a bit to evaportate, making a lovely breeding ground for bacteria, which leads to foul odor.
How to fix it:
* Shower 2 times a day
* Spray it with antiperspirant
* Keep cool
* If he's REALLY stinky, get him to wash his balls when he goes to the toilet
* Not sure if this would work, but maybe shove a block of ice between his legs? :-D