Sleepy feedings

My one month old still frequently falls asleep as the breast. He had a lot of trouble gaining weight in the beginning and was a preemie so we had to supplement with a high calorie formula for a week to get him to gain. He finally weighs over his birth weight now and his pediatrician gave me the okay to go back to nursing. I’m just concerned that he isn’t getting enough because he sleeps so often during nursing. Can anyone tell me what might be the cause for this and how we can fix it so he can be awake and nursing well?