Help me??

My 18 month have been so constipated for the longest. He poops but it's hard and look like little tinny round balls. I have tried everything the doctors suggested but I would like to change his diet and I don't know what to feed him other than what he eats now which are basically what I eat. I have tried giving him prune juice which he hates so much, apple juice and I tried giving him fruits such as pears and bananas. I even tried the water gripe for constipation. Nothing is helping. I am tired of calling his doctor. Please how are your babies doing with their poop and how is their diet?