my 1 week old

My son is one week old and I'm breastfeeding he seems to want to eat every one to 2 hours for 10 to fifteen mins. I'm afraid he's not eating enough should i formula feed as well? I fo know he has gained a pound and 7 oz since birth after he lost a pound and 1.5 ounces getting colodtrum. How do I know hes eating enough?