not gaining weight proper

This is my grandchild that we have had all kinds of weight gaining problems. started a almost birth 5lb 10 oz at birth
reflux and very slow weight gain. He has had test for refux upper and lower gi, tested for cystic fibrosis, genetic disorders . Spent 13 day in childerns hospital. the only thing that came back abnormal from the very begining was his pku. we were told it was borderline and retest and is was also borderline. We were told that has nothing to do with his weight gain. So in march at 14 mo old he had a feeding tube button placed. his weight was 14lbs  having 16 oz of pedisure in his tube feeding nightly. and 16 oz in the day time plus food if you can get him to eat. he is now  22 months old and did  weights 22 lbs until  today see the GI doctor said to reduce to 8 oz at night. so now he has lost 3lbs in 1weeks. need to know what is the next step. Also he is meeting all his mental mile stones

[Renee Radenberg]

His weight and height are both below the 5th percentile for his age. The recommendations with tube-feeding children are to tube feed at night, if possible. In your case have the amount cover his nutritional needs for the night feed so if he does not eat well during the day, he will get what he needs at from the night tube feed. During the day try to feed him regular food so he develops an appetite for food (will not be full from the tube feed formula). You can try to set up feeding him 3 meals of food so he starts to become hungry between meals and hopefully will not resist being feed orally. For the day time feeds suggest giving him a nutrient dense drink such as Pediasure or milkshake (milk and ice cream blended) in place of milk at all meals for additional calories and protein for growth and weight gain. Hoped this helped you.