Get some baby food, exactly, real human baby food, flavored "rice and chicken" dinner and mix a teaspoon of it on his wellness food mixed half and half with the science diet to avoid diarreia. If you change suddenly because of his age he most likelly will have some diarreia.
If he is such a baby , 13 wks, how do you know he has food allergies? SD is an excellent food as well, it is recomended by most vets, so it is better to keep him eating something he likes than risk having an hypoglicemia episode that will mess up his insuline levels and cause yet another more serious problem.... Are you in closed contact with the breeder? You should be... breeders know their puppies really well and have the "routines" to get them to eat and transition.... it will make it easir for you to know and i am sure that a loving and caring breeder will feell very good to have you going to them for this kind of advice...
Good luck !!!
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