It's pretty usual for preemies to sleep alot. They are still developing things that full termers had already developed before they were born. In addition, they are trying to grow, and every little thing uses energy and calories they don't necessarily have, so they make up for it by sleeping. As long as you are waking him to eat on the schedule set forth by the pedi or nicu, you should be good. Probably every 3 to 4 hours he eats, right?