Having 3 Beagles in the house we usually have more problems trying to slow them down and stop the fastest eater (Martha!) from trying to pinch everyone else's food but not all dogs are so food orientated. Many of the home boarders that come to stay with us are used to being the only dog in the house and have been allowed to graze at their food all day so are not used to eating at strict meal times.
Once you've rules out any health or dental issues that may be affecting your dogs appetite here are a few things to consider.
Read the Label Look at what you're feeding your dog and ensure you have chosen a good quality balanced dog food with good quality nutritious ingredients and stick to it, don't keep swapping and trying different foods. Ensure the food is appropriate to his age, weight and lifestyle.
No Table Scraps Avoid giving your dog table scraps or human food. Apart from filling him up he will likely prefer this and refuse his own food in favour of waiting to see what else is on offer. A lot of human food is not suitable for dogs and may at worst be toxic and cause serious health problems or may have high levels of salt not suitable for dogs. You should also avoid giving too many treats as again this will fill him up and make him look for tastier alternatives.
Strict Meal Times
Enforce strict meal times. Once the food is prepared call your dog over, ask him to sit and then give a command to let him know he can begin eating. Remain close but give him space to eat without interfering. If the food has been down for 15 minutes and he's shown no interest or he has walked off take the food up and don't attempt to offer him food again till his next meal time. Dogs will not starve from skipping the odd meal and sticking to regular meal times lets your dog know he doesn't have the option of grazing throughout the day whenever he feels like it.
Tempting the Picky Eater
Try reducing the size of meal until he is clearing the bowl then increase it back to the appropriate ration for his weight. If you are feeding a dry kibble try adding some warm water which will bring out the smell of the food and tempt him. You could also try mixing a spoonful of quality wet meat to the kibble or add some fresh vegetables. If you are feeding raw or wet food serve it at room temperature not straight from the fridge.
Introduce Change Gradually
If you do decide to change to a better quality dog food ensure any changes are introduced gradually so as not to upset your dogs digestive system. Usually over 7 days increasing the amount of the new food gradually each day.