Small baby?

Hello! I feel like my baby is very little compared to most other babies. At hos 2 month check up Dr said he was fine, he weighed 9 lbs even, but hes now 12 weeks old and is about 11 lbs. He was born 7lb 2 oz nut dropped to 6 lb 4 oz in the nicu. Im just worried he's not gaining enough. He usually only eats every 6 hours during the day, about an hour at a time, and sleeps through the night. Opinions appreciated! This is Drake!