Help me figure out what doc i need take my son to. My infant son has rectal bleeding, vomiting & crying spells?

What kind of Doctor do I need to try to find to see my son? A Pedartic Genetic Doctor an Pedatric Immunologist Doctor or some other type of doctor? Here is a good bit of info on him and the things going on with my little guy, so someone can maybe point me in the right direction. My son is 3 months old and has had rectal bleeding for over 2 months now and vomits a lot! His ped. referred him to a Ped. Gasternologist. The ped gastor. says he believes he is highly allergic to milk protiens. He had his formula switched a total of 6 times and is now on Neocate. As well as Beechnut cereal with every feeding, the Ped. Gastern. told me to stick 2 tsp in for every ounce, however I felt that was way to much cereal, for a 3 month old to be on. He is also on 3 acid reflux medications as well. The bleeding and vomiting subsided for about a week, but now it is all back again and now he is EXEREMLY fussy! I cant seem to do anything to calm him down. He falls asleep crying and wakes up crying! He has had a rough start at life, he was a NICU baby, need to be on a ventalor, cpap, nose cannale, feeding tube, had pnumonia 2x’s well in the nicu and had bronctics approx 2 wks ago. He had lung diease as well when he was born and was not able to keep his own oxygen level, they did blood gasses on him several times a day when he was in the nicu, he had yellow jaundice & had to have phototherapy as well when in the nicu. He also need a blood trasfusion when he was a week old, he had surgery for Pyloric stenonis when he was a month old. Nothing seems to be getting better for him. The doctors will try something new with him and the sythpoms subside for a week or two and are right back again. I dont know what else to do or what other doctors to have him checked by! No doctor has done blood work or any test on him. I am getting fed up with the docs. I hate seeing my baby in pain. And feel helpless when I cant do anything to make him better! I thought he may have Necrotizing Enterocolitis but I would have thought they would have tested him for that in the nicu. He has just about every sythpom of it except the budgling stomach. I mentioned it to the gastren. doc and he didnt want to hear anything about it! He is thinking it is the milk allergy as well as acid reflux, well if it is that why isnt the medication working and why is he still having all the samethings still going on! Why do they go away for a few days when they try something new for him then come back. I feel like they are using him as a guiene pig. I dont know where else to turn!
Now today he woke up crying at 615am, his crib, blanket and pjs were soaked with throw up. I picked him up out of the crib and he threw up more all over my shoulder and down my back! His last feeding was at 407am and he only drank 4oz. He had to take his medication when he got up do I gave him that at 630am. I of course cleaned him all up, changed his diaper cause he had popped! The poop had little spots of blood through, today it wasnt bright red it was darker. And the poop was a VERY dark color, hard to tell if it was DARK green or black, I used a permant black mark and made a little mark on the diaper and compared the color to the poop and it seemed to look quite the same!
He hasnt had an fevers, except a small one about a month ago. That went away in a few hours. And he is gaining weight, atleast when we were at the doctors last he was! Thats why I think the doctors dont feel this is anything to be concerned about! But, I know my baby and I feel there is something more going on with him. We are going for a second opion for a Ped. Gastnerologist, but cant get in til Feb. 2nd, however he is on a waiting list and if anyone cancels they said they will get him in sooner!
PLease help me! And him!!!
He had an ultrasound, upper gi and an x-ray, they said, they saw acid reflux but everything else looked ok!