We recommend you take a trip to the veterinarian to have your new companion's health status evaluated, Dayletta. A thorough exam will be performed to rule out or diagnose and treat any medical conditions he might be currently facing.
For a pet who is underweight but is otherwise in good health, you can focus on feeding him high-quality food to help him gain the necessary weight. You may consider feeding him an adult food that is higher in calories, one designed for active or sports-playing pets. You may also consider feeding him puppy food until he’s reached his optimal weight. There is also the possibility of free feeding your dog, that is leaving food out for him at all times until he’s reached his optimal weight.
It might be a good idea to discuss an exact portion and feeding schedule with your veterinarian. Because your vet will be able to examine your new dog and offer feeding recommendations, he or she would be the best person to give you this specific advice.
For more information, take a look at the ASPCA’s tips for feeding adult dogs.