Your snake needs a vet and medical attention. DONT try and just "let him get over it", he will die if he doesnt get the medical attention he needs, and even then he might still not make it. I had a snake in a similar situation, who got the meds and still died, RIs are serious stuff. I dont think they are contagious though, I could be wrong. Still, I would keep the snakes apart just to be safe.

If you have already taken the snake back to the store, and he dies, I would ask for your money back, because if it was a reputable store they would have never sold you a sick snake.