Does he experience numbness anywhere else in his legs?

You should be able to ride tens of miles with a flat bar without any discomfort. Is his riding position dialed in properly? Is it cold? Besides his riding position being completely whacked up I can only think of two reasons:

1) He puts too much weight on his hands AND he has tight/uncomfortable shoes or some weird pedaling habits.
2) He has some health condition and may want to check that with a doctor.