The biggest threat to this officer was definitely the crowd. It was pretty obvious that they were both physically spent early on in the fight...my heart rate was up just watching it! He had every justification to OC spray that guy or use a Taser if he had one, but that is by no means second guessing the officer. I have been there several times and it is incredibly difficult to think clearly at a time like that. Kudos to him for not giving up.
If a citizen wanted to help out in a fight like this one, the best thing to do would have been control one of the suspects arms, and then a favorite addition of mine is to control their face. By that I mean use the palm of your hand to push his face away from you and into the ground. I'm not talking about crushing his skull, or any kind of injury. Simply taking away his line of sight to what the officer (and you) are doing make a difference and there is something psychological about controlling someones head that seems to take the fight out of most people. This comes with a big warning though: if you are going for control of a hand, make sure there is nothing in it that can hurt you, and that the officer knows you are there to help (verbally and loud).
If you aren't feeling adventurous enough to jump in, watch his six for him. Even if he doesn't know you are there to help, you can keep and eye on the crowd covertly and add a measure of safety that way.