Welcome aboard to you and Hayden~ Per the pics, Hayden's body condition looks good. Be more concerned with body condition than body weight.
I checked out the Southern States dog food and I assume you're feeding the Naturals brand. Per dog food advisor, it's an average food with not a lot of meat and it's mostly a plant based food. If you're using the Advanced brand, it has a bad rating.
Southern States Advanced Dog Food | Review and Rating
Check the calcium and phosphorous levels with is the most important minerals to check on dog food. Protein is important, as well, but C & P determines bone growth.
Calcium & Phosphorous Requirements for Dogs
Dog Food FAQs: Protein
Generally, mastiffs are allergic to chicken and grains in dog food and treats. You may want to check out these grain free foods and choose a non-chicken flavor: Acana Regionals, Wellness Core, Earthborn Holistics, Innova Nature's Table, Castor & Pollux, Fromms or Taste of the Wild .Chewy.com is a great place to order dog food and they offer free shipping.
Remember to do a slow transition with changing dog food.
Slow transition to the new food is as follows to prevent diarrhea. If at any time during the transition, the pup has diarrhea return to previous amounts of food per feeding. If you are switching flavors made by the same manufacturer, you should not have to do a slow transition.
Amount per feeding:
Day 1-4 ¾ cup of old food and ¼ cup of new food.
Day 5-9 ½ cup of old food and ½ cup of new food.
Day 10-14 ¾ cup of new food and ¼ cup of old food
Day 15 Start 100% of new food
OR
You can feed the pup boiled meat and boiled white rice with canned/raw pumpkin (not the pie filling) usually start with 1 teaspoon or the pumpkin for young puppies and 1-2 tablespoons of pumpkin for pups older than 8-10 months for 4-5 days to reset their system. After the reset, start the new food.