My son grunts like crazy all the time, it's like he is trying to poo. It's not colic cause relieving gas is not a problem or acid reflux. Pls advise?

Hard to say. Since you have provided no details like age it is hard to respond. A basic fact is babies have no social conscience & don't equate grunting with anything in particular. They don't see it as anything more important than cooing.