Some people have an easier time than others saying the big "L" word. So much can factor in, its difficult to say without knowing more about him.
*He could have had a hard childhood (or an overbearing one)
*He might feel awkward showing outward emotion (does he show his affection in public, or is he bashful?)
*He might have loved someone very deeply before and was hurt by them.
*He might have lost a loved one a long time ago and since then he associates love with pain.
*He might not actually love you (harsh but possible).
There are many many reasons - but the easiest way to find out is to simply ask ... he's really the only person who can answer.
If he truly loves you then he should be able to share why its so hard to say so.