Son gets constipated a lot

He is basically always constipated. I have changed the milk, the way he eats, everything and it doesnt seem to work, it does at first but then he goes back to how he was. I have told his dr but they just say to give him the laxative they gave me for a month and i did but when i stop giving it to him he goes back to being constipated so idk why the fuck that fucking dr keeps telling me to give it to him, i don't want his stomach to get used to pooping only with help, then later he will NEED that to go. I have given him prune juice, olive oil, other juices, water, massage his belly, and he is very active he loves playing soccer. He is 15 months old. I was always constipated when i was little too, even now sometimes but not as much and it just doesnt bother me anymore. Could it be hereditary? Idk. My neices have never gotten constipated, nor my sister. My husband hasn't either ever. My mom and dad have.