Based on the video of the incident, and based on my personal training and experience, I would say this is an example of excessive force. Having said that, there may be some unknown reason I am not privvy to as to why the officer felt the need to continue striking as long as he did. I am having trouble thinking of something that would justify such actions.. for example
The subject says "I'm gonna kill you!"- This is heard about 3 times a day by the average cop on a night shift on the east side. Whether he says he will kill me or not, if his hands are empty, I am clubbing him with a baton and he goes to the ground, the next logical step is to have me and my partner put hands on, get him on his stomach, and get him in handcuffs..
If the subject says "I have a thompson machine gun under my shirt and I'm gonna pull it out and kill you!" Again... hands are empty, subject is submitting, hands on, put him in cuffs. We have Tasers, which would probably be the go to weapon of choice anyway, but I'll assume the cop didnt.
In almost every possible scenario I can think of, I fall short of a way to justify the beating short of the subject went to "Your momma.." type jokes.. which the camera clearly can't show.
Barring that, I have no problem drawing a conclusion based on the video. In almost every case officers are "monday morning quarterbacked". It's the nature of the business. We are given a great deal of authority and power, and we are subject to review of that authority and power on a regular basis.