Quoting alisha momma:" I have a appointement set for testing but right now I have to keep a log of when and how much he eats. ... [snip!] ... and her son got RSV bad and was hospitilized so dont I dont think thaty kids only got it cause they arent vaccinated."
I doubt introducing solids is really the answer. It sounds like he's eating a lot of formula per day, especially considering his weight. So he is taking in enough that he should be putting on weight. The disinterest with solids is likely more related to some sort of delay in his development, along with the lack of sitting, not smiling, etc you mentioned before. I wouldn't be pushing anything new on him until you have some answers at this point. I think you are doing everything you can be. Maybe he is just a bit slow, but will come around in his own time. Hoping it's nothing worse mama.