I agree. The heavy weight is unfamiliar- and probably scary- for him. You will need to work up to the leash slowly. As the other post says, get a thinner, lightweight one that doesn't have a heavy clasp, and let him wear it around the house. You may also want to try putting a piece of yarn hanging from his collar to get him used to having something there first.
A harness may be preferable, (my dog prefers it, even as an adult) but harnesses, like any other equipment, should be introduced slowly, with lots of praise. (let him smell before touching him with it, touch him with it before draping it over him lightly, do that before resting its full weigh on his back, do that before buckling it...)