Report it to the child's pediatrician, even if that means going around the parents. The doctor can't talk to you, but will take in the info you provide and use that going forward during appointments. No one will know that you called about it.
You can get better info from child development specialists which will carry more weight than what we moms put in writing. The American Academy of Pediatrics is one source, and you can go back through issues of Parenting magazines if you want. You could ask the pediatrician for other resources. And any good library will have resources for you - librarians are the most underutilized resource in the country!
Does he do this as "fun" or as "discipline"? You will find different sources to be usefully depending on his motivation. Here's one source and you can scroll way down to get info on corporal punishment: http://pediatrics.aappublications.org/content/101/4/723
Does he do this in public? Learn about mandated reporters and what their obligations are to report incidents like this. That can be a powerful deterrent.