Welcome and thank you for rescuing this boy! He looks BM to me. It can take a rehomed pup 8-12 months to fully trust you. He will stressed due to the environmental change, do not stress if he does not eat. As long as he is drinking water, walking and pottying normally, he will be fine.
Find out what kind of food the shelter/rescue/breeder was feeding the pup and continue to feed it to the pup until you transition to a newer food because most shelters/rescues/breeders use the cheapest food, meaning it is not very good for the pup.
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.
Generally, mastiffs are allergic to grain and chicken found in kibble. You can check
www.dogfoodadvisor.com for dog food ratings and customer feedback. Mastiff puppies should eat Large Breed adult food or All Stage food because puppy food has too much calcium causes fast growth. Slow and steady growth for a healthy mastiff. Protein in the food is not an issue unless the pup has kidney issues.
http://www.peteducation.com/article.cfm?c=2+1659&aid=652
Dog Food FAQs: Protein
Check out Acana Regionals, Wellness Core, Castor & Pollux Grain Free, Earthborn Holistics, Fromm*s, Innova Natures Table, Victor*s Grain Free or Taste of the Wild grain free. Chewy.com is a great place to order dog food and they offer free shipping on orders over $49.
You may want to check out the raw diet for your pup. Raw diet can be fed to young pups. Check out the sub forum on raw diet that has a wealth of information. It is not recommended to feed the pup kibble (one with grains) and raw food. Do your research re: raw diet and form your own opinion.
Here's a great article re: rescued/adopted pups:
5 Common Mistakes adopters make when bringing home a new dog
Please keep us posted on his progress. What is his name? Do you have other pups? Kids?
If you have kids, keep him off of the floor with the kids until you learn his behavior around children.
This is an excellent chart for children/pup behavior.
