I'm a great believer in training your dog's palate so that later in life he'll be willing to eat a wide variety of foods. If you do that when he's young he'll be more adaptable. So how about adding a little bit of this and that to his meals, doing what Jean suggested and only adding a little of the new food, mixed in with what he's used to.
For instance, my dog eats kibble, canned food, some vegetables, beef, some fish, rabbit, venison, eggs, kidneys, chicken, turkey, lamb shanks...Oh, I could go on forever.
I'm not suggesting you give him a big lamb shank, at this stage, but you could add a little lamb meat to his diet now and then.
I have a friend whose dog is a terribly picky eater, won't touch most things - it causes her a lot of worry. I sometimes wonder whether it's because he was raised on only one type of food when he was young.
Anyway, I think it's boring to eat the same food every day. I wouldn't want to do that.