How do you get through to an unresponsive student?

I'm currently working as an ELL Resource teacher.  I have a first grade student that gives no reaction or response. He is an only child and his parents are very involved with him.  When a simple question is asked he gives no answer or hint that he understands.  He scored well on PALS this fall but his classroom teacher said it took a looong time and he spoke very very softly.  Last January the WIDA Access was administered and he scored an average of 3.7.  Again, it took a long time to administer that assessment too.  He understands what is being said to him.  His work is very neat but he takes a long time to complete one small assignment.  I started to pull him from the classroom and work with him one on one to see if he opens up a little more for me.  Does anyone have any helpful tips for working with type of student?