So my son is on the verge of failure to thrive 😢😢 he lost a good half lb about a month ago and his…

So my son is on the verge of failure to thrive 😢😢 he lost a good half lb about a month ago and his Dr wanted to keep an eye on it. We go in a month later, today to do a weigh in and he lost another lb and half! He eats three meals a day plus is breastfeed on demand. I'm so so worried about him now. Dr had to stick a catheter up him to collect a pee sample and had to do 7 vials of blood to test. My poor baby, :( I wonder what's wrong with him..