my son suffers from hard stool like a rock

hi, i have a twin , a boy and a girl, they both intake same milk formula, even solid foods, but my son is constipated ever since i can remember,  he's now 2 yrs.old and a half, it would take him about 4 to 5 days before he can really cleared out, and again, you will count another 4 to 5 days  before he can cleared out again. his stool is as hard as a rock, i've raised this to their pediatrician and just advised me to give him lots of fluids, vegetables, fruits and laxative (lilac syrup) at first it really help, but since im a working mom, my twins are left with my in-laws and they're the ones who take care of them while im at office. they told me they are giving him water, prune juice..soup... honestly, he can only drink about 5oz of water each day, he'd only take a zip, i would really have to force him to make him drink more, his other fluid intakes is from powder mix juices and a bit of soda from time to time. but then, his twin sister is not constipated, granting ..she can finish her milk in no time, she eats alot than her twin brother, she doesn;t even drink water as much as his brother do..
please help, i really cant take my sons suffering everytime he urges to poops but nothing is going out from him, he'd cry, his legs turn hard and straight, he's face turn red..sometimes it is associated with vomitting....im also   constipated, but since im grown up, i can cleared out after 2 days because i can discipline myself. ..but a 2 year old child.. i cant force him to drink lots and lots of water if he really dont want to, instead, i always cooked plain soup to add on his meal and act as additional fluids. i dont want to resort on supposutories since their pediatrician told me not to. i guess something wrong with him since his twin sister is fine but i dont wanna think about that, i would like to think my son is perfectly normal, although i suspect he inherited it from me... and im normal even though im constipated, .. this is a two year old child, and as a mom..this is hard feeling for me because he is suffering from simple problem.."constipation"