Is your husband interested in raising a child to be a caring adult or is he interested in raising a child to be a criminal?
Since your husband is such a smartypants, then he can go online and do the research and look at the statistical correlation between criminal behavior and the criminals who were abused as children.
Your husband's behavior was abusive. Don't mix apples and oranges, correcting a child has no connection to what type of day a parent had. That too is also how you teach children about displaced anger. Not to mention it physically was incorrect to squeeze that particular part of a young child's anatomy. How would your husband like it if someone grabbed him and squeezed him really hard down there. In self-defense classes for women, they teach that has a paralyzing effect on the man.
It is the parent's responsibility to take care of their child, as an adult overreacting just isn't a good option. If your husband did that to your child, and you allowed it, just think of what he might do to you. Maybe he was abused as a child and so the cycle continues. The question was asked 6 months ago, hopefully you and your husband have throughly discussed that situation and it never happens again.