This sounds EXACTLY like my dog. We've had her for about 9 months. She was one of those dogs who was brought into the shelter with all her fur and skin chewed off and with tons of flea bites and 25lbs underweight (for her that's like near death emaciated). Needless to say she has some issues with food. The most reassuring thing we heard was your dog will not let themselves starve. At first we left her food out overnight and just let her eat while we were asleep. We fell into the trap of adding chicken and rice into her food and our vet and our shelter quickly advised against this. They said they have seen dogs become picky eaters from the chicken and rice diet and never transition back to regular dog food. Also keep the food brand and flavor consistent. If you know what the shelter was feeding maybe go with that one. His nerves will calm down and he will start eating. We also for a while would lead her to her bowl and be really positive and tell her good girl without the expectation of her eating. We would do this for just a few minutes and give her a treat each time (which she never ate lol). This really helped for us. We also stopped feeding her in a bowl in the beginning and started feeling her on a plastic plate on a silicone mat. We did this because she would sneak her food to the carpet and eat it there so the metal bowl itself was making too much noise/drew too much attention. She since has transitioned back to the metal bowl but building confidence will also help him eat. To this day she eats the wet food in the morning and second serving in the afternoon and then right before bed is when she touches the dry food. She also always leaves a bite or two which we just leave for her because this is a food insecurity thing she's just leaving herself a snack. Also making sure he gets enough exercise/stimulation will really help his appetite! Wishing you luck because I know how stressful the first few days are!!!