I would leave a bowl of food and a bowl of water down at all times and keep an eye for the next few weeks and see how much he is actually is eating. Then go from there.

 For starters make sure your dog is on a high quality adult maintenance diet-there are many good brands out there.  Stay away from grocery store brands and generic food.  Consult with your veterinarian about the specific brand you have chosen. 
Amount fed is based on several things--weight/size of the dog and metabolism of the dog.  I would start feeding 1/4-1/3 of a cup less than what is recommended on the bag of dog food.  The labels on most dog food overestimate the amount needed.  The total amount needs to divided into 2 equal amounts fed twice daily-breakfast and dinner.  Free-feeding ie leaving food out constantly leads to obesity-the most common problem in dogs today.  Greater than 90% of dogs cannot self-regulate the amount of food they eat appropriately.  Do not just leave food out.
The next step is to monitor your dog's weight and body condition score.  Weigh your dog every 2-3 weeks.  More importantly check your dog's body condition.  The body condition score is on a 1-9 or 1-5 scale and ideally your dog should be a 5 (on the 9 point scale) or a 3 (on the 5 point scale).  You should be able to feel the ribs easily but not see them.  You should be able to feel the spine and hip bones though they should not jut out.  Looking from the top your dog should have an hour glass waist and from the side should have a marked abdominal tuck.  Your veterinarian can help you determine and show you how to assess the body condition score.
Water should be left out all times.
Good Luck with your new friend!

 I have a shih tzu and she was 1 year in April...l feed her the Caesars soft dog food, and she eats 1 of these every day at 12pm...
    it is good to try to feed them the soft food the same time every day and l also have her hard for dog food all the time with her water dish full...she eats the soft food up all at once...and nibbles throughout the day on her hard food, and water...
   if this is the 1st time changing your dog to soft food though, your dog may get the runs at first but this will go away once it adjusts to the new food
 
l hope l was of some help to you
  good luck with your feeding!!