"儿子叫坏人打了" means "my son was beaten by a villain or several villains". here "叫" means “被”，it makes this sentence a passive form. "儿子的头让人打坏了"literally means "my son's head was injured by someone". here "让" still means "被"，so this sentence is also passive. "打坏" is to make something not working by beating it. when it comes to someone's head, it could mean "his intellegence was impaired because his head was injured"

我们一般说 “儿子被坏人打了” ， “儿子叫坏人打了“ 应该是四川方言的用法 普通话一般是不这么说的

must be _these_ two sentences

sometimes 叫，给and 让 all mean被。

the first sentence means you were hurt by a bad man,and the second means your head was hurt by a bad man.