A shadow

So I'm replacing a sick leave for two weeks. In one of my class, I have one kid that I only hear him say a word today, after seven day of teaching him!! The class is very difficult, the student has some learning disabilites(I don't really if this is how you say it, but you should understand me!) and have not much respect for the other. But this guy you don't hear him, he goes to work when ask, and I don't have to say to him a thousand time to go to work!!
That guy seems to be so trap inside his own world!! I think i always saw him wore black. Always hold his head down!! It made me sad that the most discipline student in my class seems to have a so low self esteem!!