Why do you think the poet uses the word "He" to describe the bird?

I think Tennyson is trying to do one of two things by using the pronoun "he" to describe the bird.  He could be trying to elevate and equate the bird with humans, or Tennyson could be trying to illustrate to his readers that mankind and birds (animals) are equivalent.  By using "he," Tennyson makes mankind and the bird equivalent, because the pronoun is now being used for both species (instead of "it" for the bird).  

The pronoun usage makes sense when you think about Tennyson being a Victorian author.  Victorian authors are writing their stuff at the same time that Charles Darwin published his Origin of Species.  The book was a breakthrough piece and extremely controversial, because it showed humans as completely interrelated with other animals.  The book showed that we are descended from a common ancestral species.  Tennyson's poem gives the bird the same quality as humans, because he uses a human pronoun to describe the bird.