Another trick you can do to entice him to eat is to put a spoonful of cottage cheese in his food if you have that around the house.
If you don't have that, then I've never had a failure with .5 tsp of vegetable oil.
Neither will harm your dog, the latter (In modest quantities) will help your dogs coat with softness and shine.
But if he's young and active, start with what his food is recommending and work from there if he needs less or more. Even if you feed once a day, just be sure he gets what he needs.
I've got a 16 year old, bottom level energy corgi mix who gets .5 cups of food twice daily and that's all. He's lost weight on this (But he needs to lose another 5 pounds since he was on bad food before and got fat). Right now he's 47 pounds and should probably be 35-40 max.