Explaining the loss to a 3 year old...

💟M

When we went in for our last ultrasound our son was there jumping around and yelling "look mommy look daddy our baby is so happy in your belly, he's having so much fun in there. I want him out so we can play" I dont know how to explain to him that his baby (as he calls him) will not ever be able to come out. What should I say? What should I do?