Infant son has very small appetite

My twin sons were born last October 2.5 months early. They were in the NICU approx. 50 days. One of my sons, Brandon, while deveolping normally in everything except his weight and length. He is classified as failure to thrive since he only weighs 15.5 pounds at 1 year. He has had feeding problems (not eating enough) ever since he came home from the hospital. In the NICU he would often times have residual milk in his stomach before the next feeding. I believe that he sometimes still digests his food slow because when he makes himself vomit, you can see the few bites of baby food that he had 4 hours ago (he makes himself vomit when he absolutely doesn't want anymore food).

He was on zantac for about 6 months when he first came home but is off it now.
He has had a few tests and everthing has come out ok (ph probe and swallow test). The doctors now think that the problem is behavioral.

Usually, he has 4oz breastmilk mixed with 1oz of heavy whipping cream and sometimes 2tbsp of rice cereal. This is to add calories to his diet. Out of this he will usually take only about 2 - 2.5 oz before he doesn't want anymore. He takes this very slow about 30min - 1hr. He eats/drinks extremely poorly in the morning where he only takes about 1oz at 6am and just a little better at 10am.

We started adding the heavy whipping cream about 3 weeks ago to add extra calories to his diet. He did extremely well the first 2 weeks but this last week he has dropped from drinking 28 - 30oz per day to 18-20oz per day. He has refused any baby food where he was starting to progress in this area too.

We are willing to try about anything as the regular doctors are no help at all.