Creatures abilities in mtg?

i have this card called Crypt Ripper. it's ability is i tap 1 black mana and he gets +1/+1 until end of turn. My question is can i use his ability as an instant so i can make him become something like 5/5 while defending, or can i only use his ability as a sorcery? my friend said only as a sorcery, but i think as an instant. which is it?