Stomach Noises, What Could It Be?

I picked up my little pup yesterday, but they were feeding them all wrong, now I'm concerned They were all on adult food these fellows are 7 weeks.i put myj ake on pup food but he eats like he's never been fed and drinks everthing.Then you can here his stomach like it's full of water and he whines slightly i did put him over my shoulder and did get a loud burp,is it gas or other his poo and pee are normal for this age, and he is at the vets on Friday.