Not gaining in a short period (say 3 mo) while other things are going on is mostly normal. I would say 6 mo is not quite so normal and watching with caution is a wise position. With FTT watch head growth, it goes quickly away. Height still happens surprisingly making it difficult to detect early on. We just went through this w my then 4 mo old baby boy, he's now 8 mo old.
Have you ever been tested for Celiac Disease? I ask only because there are so many misdiagnosed people in the world and you said you have UC. BTW - I don't think steriods in your pregnancy have anything to do w his not gaining weight.
CD tests in small children under age 4, are very inconclusive and that is from really bad accuracy rates in adults - blood test 29%, biopsy 33% (possibly the other way around...)
I would stay clear of Reflux meds. I would not give the meds until we had the Upper GI study and then I gave the Reflux meds, they made our FTT baby not hungry -- um, that is such a contradiction I dont' know where to begin. I had tried all the non-medicine ways to aid in reducing reflux, but they only helped some.
How we cured his reflux was with Lactic Acid Yeast, Enzymes, and Zypan (Standard Process Supplements). The Zypan I took and supposed to help my breast milk be better for him, I also take the other two.
I just has a DUH moment when the chiropractor gave me these supplements. Probiotics and Enzymes -- DUH, I gave these to my 2nd child, he turned out to be Celiac. My baby's test came back inconclusive, but I know he does not tolerate Gluten or Dairy.
Don't panic. I would nurse more, offer more solids, things that he likes. Go for proteins, fats, and less empty calorie things like rice cakes. Post back in 3 mo.
Oh if your mama ringer is just zinging away, then go see a specialist. If it isn't alerting you to a problem, then just hang tight and keep a food log, it might help determine if he is taking in enough calories.