my 3 year old son has trouble shitting

My son is 3 years old and is always constipated has always been since a baby when i bring him to his dr nothing is ever done about it and i cant seem to find a reason why he is always constipated he doesnt let it come out so he wakes up from his nap screaming standing up trying to poop he doesnt like to be touched or talked to he hides i tried one of them things you put in his butt to make him shit it out and it works sometimes but it always comes back i feel so bad that i dont know how to help him shit it out so my question is that is there anyone who went through this and knows if there is something that i can try that can help him