Who should my baby see for constipation?

My son has been sverely constipated for the past couple of months, i have been to 2 different doctors both who say it is normal and common for babies to be constipated, but my son who is 8 months, has poos so large and hard that he screams in pain and even bleeds most times from it being so big, honestly it is nearly the size of an adult poo. I explained this to both doctors who said it happens to alot of babies, but what would be causing it? They both had contradicting ways on how to fix it, one said increase solids decrease formula feeds, the other said to do the opposite and neither worked, i have been adding prune juice daily to his night bottle (on the advice of my mother-in-law) which has managed to control it, but as soon as i stopped the prune juice he is constapated again. What specialist should i have him referred to? A nutritionist maybe? I have had it with gp's. It is really distressing both for my son amd myself.