It's a combination of metal lacing his bones, intensive training, obsessive exercise, and admittedly some memory of how much he could lift before the serum was nullified. He can do it, but he's going to regret it later. Ice, aspirin, and a long bath will be in order after this.

"People have been calling in reports of this thing for days. And Glouchester is pretty used to weird shit by now. If you want to cover its head, I guess, but. I think we'll be okay."

He gives it a nudge with his rifle butt, and when it doesn't react at all, he offers the rifle to Caitlyn to carry for him.