What did Dustin Hoffman do to Justin Henry?

Meryl Streep with possible fellow victim Justin Henry.

Dustin Hoffman "helped" Meryl Streep with her acting in Kramer vs. Kramer by slapping her in the face before an emotional scene. Justin Henry was seven- or eight-years-old. You don't think he needed even MORE help with his acting? He did a lot more crying in the thing than Meryl Streep did.

What did that monster Dustin Hoffman do to Justin Henry?