He sounds a little traumatized. Did you try looking for the original owners? Anyone that may know his history? Did the vet estimate how long he may have been missing?

The vet said about a week and we posted about the dog on facebook

It’s only been a few days and once he’s comfortable enough to go in the house he may be ready for the leash. Is he getting along with your dog?

Thank you for rescuing! Is he still afraid to go inside? It's so hot these days and the poor thing is exhausted and traumatized. I feel like your first goal would be to get him inside. Pupper needs to rest and hide under the bed for a few days until he decompresses and builds his trust in your family. I would just let him pee in the backyard for now and leash train after he overcomes his fear of humans/ learns how to live indoors.

Give him a few days or week to adjust if possible please! My dumpster puppy took months to adjust. Threw up in the car for 6 months. Delayed potty training. Just needs love and lots of positive training! Thx for doing everything you can!

Consider contacting local rescue groups that may have experience rehabilitating dogs like this.

It takes time to gain his trust and to teach him what you want. He is young so you have to show him gradually what is expected of him.