untreatable constipation in Toddler

Hi, my now 30 month old toodler son has suffered from severe Constipation literally from within a few days of his birth. i will need to give you some of his medical history so you can get a good picture so please excuse the length of the thread.basically his problems started when he was only 10 days old. maybe even earlier because after thinking back i remember the paediatrician mentioning that he had a very distended belly and asked if he had passed the first stool, which he had, though i cannot remember exactly how many hours old he was.when he was 10 days old he started bleeding from his anus and we brought him to the hospital where he stayed for a few days and it was put down to a milk allergy. he was put on the formula Neocate. the bleeding stopped but the constipation has remained since then and to this day. he was put on lactulose solution starting at 5ml twice a day and then moved up to more. he is now at 20 mls twice a day and his stools are still rock hard and he has yet to make a poo without the help of an enema, which we have to give him roughly every 4 days because he is in a lot of discomfort because he keeps being bound up by the hard poo's. i have been back to the doctors with him up-teen times and everytime we are just told to increase the amount of lactulose and to give him an enema every 4 days if he produces no poo. at once stage he was on 10mls lactulose twice a day and also movicol paediatric twice a day, both at the same time for about 6 weeks, with no softening of poo or other improvement.we are at our wits end. at this stage both me and my husband are convinced that our son's problems are more than just constipation. please advice.