There are many relatively minor reasons the dog might be eating less. These may clear up on their own, but you can still try to help them through it. If none of these issues fit your dog's situation, you may need to contact a vet or consider medical causes.  Some dogs get motion sick when traveling. Others have trouble eating in new surroundings following a move. Some dogs dislike being fed in an uncomfortable situation. Try to keep the dog's bowl in the same place, at a comfortable height, and away from other pets that try to eat the dog's food. Dogs may react to the departure or arrival of another pet or a family member. The cause may even be something small, like changing furniture or having house work done. Some dogs don't eat because they want attention from their owner. If the dog ignores its food and looks to you for attention, ignore it. When you feed the dog, put down its food for 10 minutes, ignore it, and throw away whatever doesn't get eaten. The dog may simply happen to be picky about the type of food he eats. Most dogs would much rather eat that piece of steak and mashed potatoes than dog food. He'll love you for giving it to him, but over time it can make him a finicky eater and a beggar at the table. Keep careful watch on your kids — they aren't always the best about cooperating with this rule. Regular exercise will stimulate your dog's appetite and encourage him to eat more. To really boost the appetite-improving effects of exercise, take him for a walk before each meal. It won't take him long to associate the walk with mealtime, giving him a positive association for both activities.  Although some types of dogs require more activity than others, ideally, your dog should be getting exercise every day or at least several times a week. If you aren't capable of participating in physical activity, there are plenty of other ways to have your dog exercised for you. Take him to a doggie day care center, hire a dog walker, or go to a dog park and let the other dogs tire him out.

TL;DR in Vietnamese:
Tìm nguyên nhân. Giảm cho chó đồ ăn và thức ăn thừa của người. Cho chó tập thể dục.