He is only one of the boy students who speak English well.

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!

A这里的who指the boy students,所以动词不是单三.
B这里的who指He,所以动词是单三.

1A
2B

1.A
2.B
因为有了the所以第二句指的是一个人

A
B
第一个only one指的是独生子的意思，译为他是那些英语讲的好的男学生中唯一的独生子．
第二个the only one是唯一的意思，译为他是那些男学生中唯一英语讲的好的．

a
b