The tough part about his suit against his employer is that he was almost certainly operating outside the scope of his responsibility when the altercation occured. Very likely that his directive in this situation was to verbally request for the skaters to leave the property and if that did not happen then he was to involve the police in the matter. His employment contract/training had to have specifically dictated that he not get involved in a verbal or physical altercation with anyone on the property and to deescalate any such situation in order to avoid exactly this scenario.
Even though I imagine that it stated in his contract that he must only verbally request the trespassers to vacate the property, I bet there is a culture amongst the security guards (as we see with police) that they must go above and beyond their station and do everything in their power to defend that property. And if that way of thinking is pushed by someone in seniority to that security guard, such as a manager, then that manager as a representation of that company puts the company into blame. The injured security guard in my opinion has the right to blame the company and to sue them for putting him in danger.
Companies will absolutely reward employees for going above and beyond their roll if it means the company benefits from the action. So this culture or rewarding employees for going outside their lane exists in all industries. However if the action of the employee does not benefit the company or even harms the company in any way then that company will default to the terms of the employees contract to the word and accept no responsibility.
Nobody deserves to get injured in any situation especially when they are selling their time to a corporation. The skater(s) need to be punished for the actions they took against this man if a jury of peers can find them responsible given all the evidence they are presented with. We do not have all the evidence, we are just throwing our worthless opinions around in pure speculation.