I'd say pee. If you had a picture without your URL stuck on it we may be able to tell from the appearance of the arm pit region. If it is sweat I would also expect to see moisture in that region of the shirt.
i think that's pee... it doesn't look like a sweat for me... coz if it's a sweat, it should be scattered all over the shirt or not totally all over, but its scattered..
Based on the kids location you'd assume pee. However the picure IS in the desert so I"m going with Sweat...
yeah, therefore it makes it sweat....plus the guy is walking in a very "desert" looking place...with a kid on his shoulders...the upper back is one of the first places to sweat, next to the palms and head...