My 10 week old baby is eating 8 oz at a time?! and some times wants more?

My baby is such a good baby, and usually only crys when he is hungry. I have a 2 year old as well, so I know that he is really hungry and not just wanting to suck. He will chew and grunt on a pacifier until he gets annoyed and spits it out. and will cry until you feed him. I've been burping him after every 2 oz because he has a little bit of acid reflux to help cut down on spit up catastrophes. But he will eat .. and eat.. and eat... He usually takes 8oz every 4 hours, and some times wants more but I try to just hold his py in his mouth and rock him to sleep, but some times that doesn't work and I have to give him a couple more oz. My first son didn't eat 7 oz at a time until he was about 6 or 7 months old! I don't even know where he's putting it all! He's actually a little bit under weight and is not chubby at all except for the usual baby cheeks. He is solid muscle, and I think he may be needing more because he has so much muscle, but I'm just baffled! Has anyone else had a 2 month old baby eat this much? Any tips would be great. Thanks!