Any Axis lock Benchmade.
It can be opened with one hand (either hand) as easy as a switchblade. You can either use the thumb stud hole to slide the blade open normally, or flick it open. Axis lock knives flick open so easy they almost do it telepathically.
Axis lock knives are also VERY easy to close one-handed, and with either hand. Just pull back on the Axis lock with your thumb or finger (and with either hand as you can unlock it the same way on either side) and let gravity close the knife. You can also help it along, but gravity will do it by itself.
It is basically a switchblade that fires out, and fires in, but without actually being a switchblade.
Liner lock knives can be opened and closed with one hand, but the opening is not as smooth and easy as an Axis lock, and closing a liner lock with only the left hand is a bit hard. So if he has his right hand, a liner lock will be fine, but an Axis is prefered. If he has his left hand, an Axis lock is head and shoulders the best choice, IMO.
Most BM Axis lock knives also have a clip that can be moved to the other side for left-hand carry. Don't overlook that detail.