11mth old constipated even after prunes. Help!?

Okay my son has had bowel problems ever since he started solids at 8mths old. He eats rice cereal twice a day w/ veggies or fruits (pureed). The doctor said at his 9mth ck up to give miralax once to twice daily. UGH! Hate giving meds. It helped, but only given a few times when I know he really trying to go. He drinks 4 oz of apple juice a day. Last 3 days he had a whole container of prunes w/ his cereal, and prune juice. He was having very firm stools. (most the time like "rocks"). Today he woke up with pebbles in his diaper. After all those prunes you would think it would be a little softer. Don't know what to do. Anything he eats makes him constipated. He is also fed. Also at his 9mth ck up I was explaining this to his doc and he said"Hmmm, just give miralax"! IS this normal?? Should we see a GI specialist?