Question: how to gain weight for dogs?
Answer: Choose a higher calorie food. Select a food that is higher in calories and protein like what you would offer a canine athlete. ... 
Feed small meals throughout the day or consider free feeding. ... 
Supplement meals with snacks. ... 
Use wet or fresh foods. ... 
Picky eater. ... 
Stress. ... 
Illness. ... 
Old age.