As long as he put on weight and doesnt look underweight I wouldnt worry, it may be that he sees and smells what your other dog eats though. One of my danes she used to be very picky as a puppy too, and it was to the point where she did not put on weight. I had to put water in the food and "swell" it then make little meatballs of it and roll them over the floor, she chased them and ate them, thankfully we didnt have to do that for too long, let me tell you, it takes an awful long time to roll little meatballs three times a day. Good Luck
Marit