There are commercial dog foods available such as royal canin Doberman food useful as the dog food to help gain weight. However, ensure the food contains the first ingredient as meat and this is mandatory for Dobermans as per the USDA guidelines. Assure to buy the best dog food for Dobermans that have:
royal canin - Doberman Forum : Doberman Breed Dog Forums
I know that commercial food choices in India are not like they are in the USA - Royal Canin is probably one of your better choices. I would say that at 5 months, you can switch him to adult food. Dobermans are not a breed that stay on puppy food for a long time.
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