Marguerite,"boys being boys" is the most lame excuse a parent can use for bad behavior. Where in the Y chromosome does it say brutality is acceptable if you have a penis?
Every time someone says that to me I seethe. Parents who use that excuse are morons. It is positively the largest load of horse dung excuse made.
I'm sorry for your son. It breaks my heart. Brutality is cruel and it diminishes your son's self image. Civilization says we have moved passed pack mentality. Attacking the weakest in our group is a sure sign of cruelty.
Many hugs to your son and to you.