As a former Jacksonville FL Sheriffs Deputy, I can say that it is NOT in any circumstance to use proper use of force. Once the suspect is handcuffed, the officers should have carried him out. There was never any threatening behavior from the suspect, so even to tazer him the first time is questionable. No, the video does not show the entire incident, but the statements from the witnesses do. He was leaving, when approached by the officers. They should have let him leave.
As an officer of the law you are given training to subdue in many forms. You are tazered, sprayed with mace, and even have tear gas training. Of the 112 officers that received this training, only 13 were incapacitated, after being tazed, for more then 30 seconds(mind you these were all grown men, physically conditioned, and expecting to be tazed). This being said,he was not in this condition, and it is feasible that he was not able to get up on his own after just his first jolt. Another thing you a trained for is to respect medical conditions. this may sound like a load of crap, but read on.
Now as a practicing civil lawyer, he has a very strong case for not only excessive use of force (police brutality), but also false arrest. He is not the only one who is in for a chance to sue the dept., because the kid that was threatened also has a good case if he chooses to pursue. I've seen big awards for less cause.
The last case my firm brought up against a police dept. was in Albany NY (I'm licensed by NYS Bar Assoc.). The case involved an epileptic man that was pulled over for a broken taillight. when the officer approached the car the man informed him that he had epilepsy, and asked him to turn off his lights. The officer refused, and under stress, along with the strobe effect of squad cars lights, the man went into epileptic seizures. That case was settled out of court for over $300,000. The officer never touched him, never used force of any form, but failed to respond to a reasonable request.