[sbaglio]

I would speak again with your pediatrician. Your son seems to be taking a ton of formula for his age and size, and the calories don't seem to be translating into tangible weight gain. Not having examined him or knowing anything about his medical history, the weight loss from birth until now seems excessive. At his age (roughly my DD's age), he needs more calories from food sources, and less from milk alone. Has the ped mentioned FTT? As Fran suggested, your pediatrician can send off tests to check various nutritional parameters. Best of luck.