Honestly my (untrained) opinion is that if he seems healthy, happy, and is doing well in the diaper department, it's ok to not worry about weight gain (unless of course there's something they're seeing that I'm not). Will he take pumped milk from a bottle or have you not tried? You might want to try smaller bottles too so you're not pouring out all that formula -- or could you save it for the next feed (I don't know anything about ff...)? Hope you have some luck soon!