For a 2.5 month old Golden Retriever puppy, Royal Canin Starter food will be great atleast for the next 6 months or so. A puppy's stomach is weak and requires food that is easy to digest. Royal Canin starter food has been specially designed to meet the nutritional requirements of growing puppies. Cerelac is food for human babies and the nutritional requirements of a human baby compared to a puppy are very different. Cerelac can also cause change in the stool colour and upset stomach.
If you feel that Royal Canin is an expensive food, you may choose starter foods from other brands such as Eukanuba.