Need advice

This little guy here was walking normal a few days ago and then one morning I went to let them out ( he has four siblings all frenchies) and he didn’t want to step down onto the patio and when he did he let out a soft yelp. So I carried him to the backyard he peed and pooped but won’t use his back legs now. He can stand sometimes but now he will hardly move. He is still eating and going to the bathroom. He was sitting in his poop this morning because he didn’t move. I know I’m going to take him to the vet but please anyone with run ins with this. TIA.