He is simply an enthusiast.
"It is good for mankind that there should be some enthusiasts.

I grant that it would be an unpleasant world if we were all enthusiasts, but the sight of a man like him throwing his whole life and energy into his work, and wearing himself out trying to lessen the evils he sees around him, ought to do good to us all.

Look at these boys," and he apostrophized Wilson and Richards, as they appeared together at the door.

"What do they think of but amusing themselves and shirking their duties as far as possible ?" "Oh, I say, Doctor," Wilson exclaimed, astonished at this sudden attack, "what are you pitching into us like that for?
That is not fair, is it, Major?
We amuse ourselves, of course, when there is nothing else to do, but I am sure we don't shirk our work.

You don't want us to spend our spare time in reading Greek, I suppose ?" "No; but you might spend some of it very profitably in learning some of these native languages," the Doctor said.