Who is right?

My baby is 5 months old and will almost start solids. My aunt tells me when i start giving my son solids i should replace 1 or 2 5oz bottles of milk with only baby food cause he wont get full with only milk and also to start giving him water. I am a ftm but that does not sound right to me. Who is right? She keeps telling me i dont listen and she knows more and he wont get strong if i dont replace milk for soilds. It just doesnt seem right to me but im not sure how to add solids to his milk feedings when he starts. Advice please.