I'm wondering how they have classified him as failure to thrive? My daughter was lactose intolerant and it took the first six months of her life to rule out everything else. (she has since grown out of it) However she still is a very small kid on her own curve well below the percentiles they give. But she is thriving just fine. At 21 months she was 32 inches and 20lbs. I know girls and boys have a different curve but... Has he been maintaining his own generally parallel curve to the norm or is he dropping?
I had a lactation consultant call and harass me that I was not a good mom because my daughter wasn't getting enough because I wasn't feeding her long enough. (I know this to be contrary to the truth because my second child is thriving perfectly well (he's a huge little fatty) and eats even less time than my daughter, my milk comes out fast and they get a full feeding in a short amount of time)My daughter just couldn't tolerate the milk and puked it all up, I mean projectile vomiting.
I remember it was a worrisome time for me but I needn't have been so worried once we switched her to soy formula. My daughter was and is plenty vital and active, she is bright and talented, she is just tall and skinny. Since being able to drink milk her weight has finally climbed up to just below the 5th percentile while her hight is in the 75%. So she's still on her own curve.
While there may be something, don't stress if he is mentally active and eating well and developing fine and it sounds like he is. What are the doctors freaking out about?
As parents we tend to worry esp when a doctor alerts us to something but you are his mom, listen to your intuition.