1.He is only one of the boy students who speak English well.
'Who speak English' is a relative clause modifying its antecedent,students.
2.He is the only one of the boy students who English well.这是一句模糊不清的英语.应当尽量避免.relative clause 关系子句应该尽量靠近它所修饰的前行词 antecedent.
我刚刚和英美人士交换过意见,连他们都感到困惑.
较正确地说法：
He is the only boy student who speaks fluent English in the class (or school..etc.).
如果你是老师的话,应该尽早删除此题.
Hope this help.
Cheers
Note:本人已在海外生活了数十载.为了慎重起见,我与一些美国人和英国人交换过意见,信不信由你.教育乃百年大计!