ADDED: Jim Webb's writing career is clearly more substantial than the usual "[p]oliticians who dabble in fiction writing." And the line involving the father and son -- shown out of context at the link -- is not part of a sexually titillating scene as I discovered by going to Amazon and use the search-inside-the-book function to see the line in context. Click on the comments for more discussion.
UPDATE: Webb explains the scene:
"It's not a sexual act," Webb told [radio host Mark] Plotkin regarding the "Lost Soldiers" excerpt. "I actually saw this happen in a slum in Bangkok when I was there as a journalist."
"The duty of a writer is to illuminate his surroundings," he added.
Coincidentally, a Cambodian woman in Las Vegas is facing sexual assault charges for performing a similar act on her young son, according to an Oct. 14 report in the Las Vegas Review-Journal.
The article quotes an office manager for the Cambodian Association of America, who described the act as a sign of respect or love.
"It's an exception," Thira Srey told the Review-Journal of the practice. According to the report, the act is usually performed by a mother or caretaker on a child who is one year old or younger. In Webb's novel, the child is four years old.