Need help

So my baby was born on the first & ive been exclusively breastfeeding. He weighed 7lbs & 7oz at birth. When we went back for his check up, he lost quite a lot of weight, he now weighs 6lbs & 4oz. I feel terrible cause he’s not gaining weight but he never seems to be starving or anything. He’s having enough wet & poop diapers. He’s usually on the boob for about 45 mins sometimes an hour, that’s including both breast. I burp him in between switching boobs & he’s doing pretty well. I’m just really concerned about his weight loss. I really don’t want to give him formula but I’m wondering if that’s the best option. I breast fed my daughter & I remember this happening, where she wasn’t really gaining weight her first week but then we did pretty good for 8 months. Someone please give me advice 😢