I'm with you. BOTH kidneys should be removed the moment he's found, one for his son, one for someone else.
Come to think of it, his son could maybe use both kidneys.
I'm thinking that we find him - take his kidney - fly it to his son who is waiting for it - then leave him in Mexico with a single kidney and a bounty on his head. If there were a way to put the other kidney on ice and save it for the kid, I would advocate taking them both up front.