My premature baby is finding it difficult to poo?

My little baby is finding it difficult to poo, but I'm not sure of the best method to help him go. His gp and neonatal nurse said to give him orange juice. The gp said give him it once a day, if this doesn't work twice if not three times a day, as you cant overdose on orange juice...however I'm worried about it giving him tummy ache. I have tried rubbing his belly and moving his legs in a bicycle motion as well as giving him a bath, it used to work...but doesn't seem to anymore. His health visitor said give him an ounce of boiled water, this didn't work either, she also said orange can gripe his bowels. She also said don't give him brown sugar because this may make his blood sugar levels go up as he is prem, (he was born at 29+4 gestation). I'm confused as to what is the best thing to do. His poo isn't hard pellets, it just seems to get lodged and when he does go the consistency is sticky. Advice would be much appreciated :) xx