Baby throwing up/ spitting up every meal 😕

My baby boy is 9 weeks now, and since yesterday morning everything he eats he throws right up. It's a lot more than a typical spit up for sure but not an entire feedings worth. I took him to urgent care last night because he was so dehydrated his soft spot was caved in. They did some tests (all negative) and then said it's probably just a viral bug and it'll pass, had me give him pedialite which he did okay with so they didn't have to give him an IV of fluid. He's stable now but still throwing up, I'm just wondering if it's really a bug or if something else is up. You'd think if he were sick he'd be fussy but he's totally content just can't keep much down. I'm just a worried momma, I want to know why he can't keep most of anything down. And also, how do I know when to give him pedialite and how much is too much? He had 4oz at the hospital, 2 oz last night at home and 2oz this morning at around 8am.