Why would my son have been sent to the hole for cussing out a guard? Could he have gotten hurt during this?

Well, for one, he is in prison and in prison the guards are there to keep order. If an inmate is out of order and can't behave while locked up, they will isolate them away from the general population. If they continue to act out, they will do their time in solitary - this is hard time, emotionally. He will not get caught up in anything that will get him hurt because he will be away from any conflict while he's alone.