can my son come in with US tomorrow to the 3d ultrasound?

depends on where you go but im pretty sure it's fine

It depends where you go. Some places don't allow kids

it's just a 3d/4d ultrasound place

they mostly expect family. i had both the babies grandmas and step grandpa's and the babies uncle in the room

usually if it's a private ultrasound place, family is allowed, as long as someone is there to watch the child it should be fine.