It looks like he uses a smile pattern (a la 1H BH) for his takeback, and then in his forward swing, he executes a normal FH swing, across his body, with an extreme, extreme grip. One of Sampras's FH variations is actually like this (esp. when he wants to hit a sick running CC shot), although obviously with an Eastern grip. During the transition from height of takeback (which is also the end of his takeback) to the forward swing, he lets the forearm passively supinate to open up the racquet face.
This style can give you some of the benefits of a straight-arm FH. First, it expands the contact zone of an extreme grip. Even with a HW grip, you can hit shots below the waist with authority, and still hit back shots above shoulder level. Second, it flattens the slope of the swing, so that lets you drive through shots and hit flat, regardless of grip. Moreover (and unlike a straight-arm FH), it does this without pushing the POC farther forward relative to an equivalent WW swing.
It also has disadvantages. First, it doesn't load the shoulder as well as a normal takeback, and so pure racquet speed will not be as high. Second, because there is some necessary elbow deviation to make the transition from a smile takeback to a regular forward swing, it constricts the WW action of the stroke. Now, that seems counterintuitive given the video clip, BUT if you've seen a proper WW FH executed with a Hawaian grip, the racquet rotates almost a full circle.
However, weighing in things, he probably has a very, very heavy shot, and he probably does well with both flat bashers and topspinners. Also, this person seems to have both this kind of FH and a more normal one with his grip. I'm not sure if he plays angles that well, but his shot comes in lower than a person is used to and it kicks up really high.
Actually, what I saw looked quite good.
