Failure to thrive?

Has anyone had any experience with FTT? I'm not sure its what my son has, all i know is he was a champion eater until mid december, when he just started eating about 1/2 what he was previously consuming. In January he'd lost 2lbs since Nov, so we switched formula, he started eating normally, but then we confirmed he was allergic to the milk protein, so we switched to soy. He's been on that for 4 weeks. The first week was great, then he got RSV. He was hospitalized, but has been out for 10 days now and is happy as can be, but still won't eat more than 20oz a day. He should be eating more like 30-36oz a day. I cannot get him to take more and i'm starting to worry that he has FTT, as it can be caused by illnesses as well as GERD, which he's had both. I weighed him today and he's not gained or lost anything since the last day in the hospital, but he should be gaining now, and still has not doubled his birth-weight. I have a message into the doctor, I just wanted to gather any info from anyone else who may be experiencing this.