Baby losing weight

I’m feeling totally blind sided. My baby is peeing and pooping well but went in for a check up today and he lost 6 ounces in a week. I am EBF and thought I was doing well with him. The only thing I can think of is that he is maybe not really sucking or getting too sleepy and seems full. I feel really frustrated and like I’m letting him down. I tested how much I produce by pumping and got 6 ounces so I know it’s not a production thing. My son is currently 12 days old. Any advice on increasing his calories and weight with out supplementing? Thanks!