Do let us know how the U/S report went. Being back at his birth weight after 3-4 weeks is not a good sign. I follow the Facebook Group on this and the story is so common. Positive side is - operating on a baby is no small thing, so wait and consider alternatives.
The easiest-to-spot signs for parents of PS babies are significant weight loss, puke without bile (greeny fluid from passing the liver), hunger immed after vomitting, no or minimally soiled nappies, lethargy, dehydration (no wet nappies, wrinked skin, sunken eyes if it gets that bad), visible muscle ripples over the stomach with vomitting, and "the pyloric olive" (the swollen muscle as a 2 cm firm lump) able to be seen or felt when the abdomen is relaxed (eg during feeding) - see the web for this kind of info.
Again, very very few babies die of PS or the surgery now, but it usually puts the parents through a rugged time and some infants have a few ongoing problems like reflux until they grow out of it.