Concerned about son not drinking and eating enough

My son just turned a year on June 16th.  He always ate very little; however, over the past month, he is now giving my cousin who watches him during the day a very hard time with drinking formula, water, etc.  We take him to the doctor quite frequently due to our serious concerns about him being underweight.  His one year appt. was last Thursday and he weighed 18lbs/7ozs, we took him back in yesterday and he now weighs 18lbs/1oz.  It was 90 degrees out yesterday and my little guy did not have one ounce of water, milk, formula ANYTHING with my cousin.  However, when we brought him home, at dinner he had 13 ounces of formula/milk with me!  Prior to a couple of weeks ago (exactly three weeks), he didn't have any issues with drinking with her.  The doctor says it is behavioral (not mechanical) since he is still drinking with us.  We are waiting for bloodwork to come back for celiac disease and we are waiting to meet with First Steps for advice on getting his appetite up.  My little guy doesn't have any aversions to food nor any texture issues.  He never has been a big eater.  He was diagnosed with silent reflux at about 8 months, placed on prevacid and everything was wonderful!  At our 1 year appt., we took him off the prevacid because he isn't showing any symptoms any longer with vomiting in his mouth and swallowing it.  He can have two small scoops of mashed potatoes and three noodles and he is done.  As a side note, he does have Duarte Galactosemia and we haven't had him on milk products for the first year of his life (he was and is on soy formula).  We are slowly introducing milk products into his diet and the transition has been fine.  We do also have an appt. again with his metabolic specialist at Riley Hospital so we can maybe get some hands on advice to try to get his appetite increased and to make sure he doesn't have any issues.  He is on target developmentally and you would never know he has an eating issue. ANY ADVICE would be so greatly appreciated.

[Renee Radenberg]

I have to agree with your MD that it maybe behavioral issues because he drinks fine for you. He is a year old and can take his liquids from a 'sippy' cup. Suggest he start eating finger foods and feeding himself with his fingers. He is underweight, suggest making soy shakes (soy milk and soy ice cream blended) or soy smoothies (soy yogurt, and frozen fruit blended) for additional calories and protein to help his gain weight and grow. Thank you for your question.