Castor oil for toddlers

Is it safe? I've read from people online not to do it, but the bottle has children's dosage on it. My bf's son gets bad constipation and nothing helps. He eats generally healthy, drinks apple juice.. when he does poop once a week, it sounds like he's going through sheer torture and fills 3 diapers. He's 3 and is potty train but refuses to poop on the toilet because it hurts him so bad that the only thing that helps him is to run around and scream his head off and cry. Then when he finally does poop it looks like cement and smells AWFUL. Sometimes I can't believe how much comes out. Probably because it all builds up over the week. Has anyone used toddler constipation medicine that worked? We actually tried that before too and still nothing. I thought maybe castor oil would do this trick since it's probably stronger.