He 100% mimes. I have seen them a couple of times in recent years and it is obvious. I also saw them in 2015 and he was not miming... his voice was horrific. I used to live around 2 blocks from the stadium and when I came home my wife was laughing at the wailing of Paul's voice coming downwind from the stadium. They use a live backing track, it is not even in doubt.

And by the way, it absolutely takes away from the experience. KISS are playing Monsters of Rock in Argentina in a couple of months. I am going to see Deep Purple, Helloween, and mostly Avantasia. I think Scorpians are on the bill too. KISS (I think) is headlining and I will be going home before their set.