A fit can describe him, 7 letters
He is, in fact, a zealot, 7 letters
More than enthusiastic, 7 letters
One who's more than devoted, 7 letters
Owner of a complete set, 7 letters
A person motivated by irrational enthusiasm (as for a cause), 7 letters
Santayana defines it as redoubling your effort when you have forgotten your aim, 10 letters