Think about it: he's clearly a defender, he's powered by the ancient spirits of his planet, he draws attacks from his foes...
And how's about some ranger multiclass for a few "beast" powers?He wears light armor...
I would say Tom Bombadil is a Warden. "Master of wood, water and hill," is unaffected by wearing the One Ring, commands the forest-controller Old Man Willow, and Wikipedia sets up a series of quotes to describe Bombadil as "the warden or avatar of the nearly invincible aspects of the planet itself".
Bombadil strikes me more as a controller/leader type, which'd make him a Druid or Shaman, whereas Beorn is the down-n-dirty fighting Warden.