3 week old constipated & formula similac sensitive!! HELP!!

My son has been pooping little hard balls of poop and is very constipated . He's a preemie and was born at 36 weeks. 4lbs, 7oz. We have probed his little booty with a qtip and baby Vaseline and that helped him push out a few more pellets, and we tried the baby move stuff with the prune juice in his bottle and still nothing . Please help. He's so fussy, gassy, pushing and pushing and crying. I need your help mommys! My poor guy is in so much pain! Thank you so much !!!!! Please help