I think it's drool from his mouth on his stomach. But the front of his boxers is damp round his bulge, so maybe he came inside them. And when they carry him out of the house, the wrinkled back of his boxers looks like he shat himself a bit.
I wonder why he'd dropped his trousers so he only had his boxers on as he did it?
I love it when guys are carried over the shoulder like that -- it's so easy to pull their shorts down, pat their ass, and put my hand underneath to give him a squeeze.