How to get my 4 week old to eat more per serving... FTM of a healthy, growing 4 weeks +3 day baby. He’s fed formula and breast milk (nursing and expressed)

Lately he hasn’t been eating as much at each feeding (like 1.5 to 2 ounces) but eats constantly. He tends to spit up a lot and also acts as if he’s really uncomfortable while eating sometimes, even though I know he’s very hungry. 

Is there any way we can help him to feel better so he can eat more? He doesn’t sleep longer than 1.5 hours (and this is VERY seldom....like hardly ever). We’ve tried Gripe Water and that does seem to help, so should we continue??

Any and all advice would be GREATLY appreciated. This new Mama could use the help