I think he could. For certifiation he could pass his checkrides as long as he shows "demonstrated ability." I've met private pilots who were missing entire limbs. As far as commercial flying goes I personally would consider it serious unless he was missing his thumb and forefinger. Most airlines use "ability to reach and operate all controls" as the standard to be judged. He may face a bit of an uphill battle because of mis-perceptions but nowadays airlines in the U.S. are very careful about arbitrarily rejecting someone unless they can clearly show that his missing fingers was a handicap. (Many airlines such as United no longer even require a standard for visual accuity as long as you can see 20/20 when you are wearing your glasses.) Several airlines have lost costly lawsuits stemming from bias issues... height/weight ratios are no longer present, again as long as you can operate all of the equipment safely.
Don't let ANYONE keep you from pursuing your dreams. Many people told me I couldn't be an airline pilot because of what they perceived as obstacles (I was poor, and I wear glasses). The truth is they just didn't know, besides whats true today may not necessarily be the standard tommorrow. I know three people who flew in the military WITH glasses (2 F-15's, and 1 f-16)...supply and demand.
Heck, I've flown with a couple of captains who cant seem to work one thrust lever with all their fingers!
Go for it!