Can you add karo syrup to formula to relieve newborn constipation or must it be mixed with water?

My 12 week old son gets very hard stools and its painful for him so my doctor said to give him a bottle with karo syrup mixed with water. He takes it fine but then it makes him go longer without a formula feeding b/c it fills him up. Then it makes his later feedings very close together and it throws him off schedule. My question is, can I just add the karo to the formula, or must it be seperate with just water?